I started a random character in NetHack, and got a female monk and a dog. I know this game is hard, so I would appreciate some hints from more experienced players. ^ ^'
My friend was talking about this game, so I got interested in playing it.
I know I need to be carefull with what I eat or drink, some items can be cursed, and I need to get food or I'll die of starvation.
Any tip, people?

Oh man... sorry... if you played [url=www.Adom.de]Ancient Domains of Mystery[/url] i could give you hints, I never really got into Nethack...

Speaking of food, if your pet won't eat something that it normally should then that means that the item in question is either poisoned or rotten. So if your dog will not eat something (occupying the same tile as the item of food in question) then it means that you shouldn't eat it.
When trying out unidentified wands ALWAYS stay as far away as possible from any walls because projectile spells WILL bounce back at you and KEEL you.
Pray in-game and IRL.
Make sure you can learn and use Identify it will prevent problems later on.
Don't kill and sacrifice your pet. Or eat it for that matter...
Always kill any bats, lizards, iguanas, wolves, dogs (other than your own, of course), and other animals and make sure you can store the food you find either by canning (not sure if you can make canned food in the original NetHack or not but whatever...) or using a cooler.
Don't eat zombie corpses...
If you do end up wearing an item that is cursed, try dipping it in a fountain. You may or may not remove a curse that way.

Thanks for the hints!
The same thing works for cursed items right? My friend said that sometimes the dog will try to stay away from some items, because they are cursed. Is that true?

This is good advice, but be careful about eating corpses because you get an alignment penalty when you eat meat, which is a bad. But the penalty fairly small, so it's ok if you eat meat only once in a while.
Monks become useless when wearing body armor. Instead, wear a robe if you can find one. Boots, helmets, gauntlets, etc. are safe to wear.
If you see a "grey stone" then kick it. It if cannot be moved (you might hurt your foot but that heals soon enough) then it is probably a loadstone - do not pick it up ever. If it goes flying then it is safe to pick up (unless it is a luckstone, as those are cursed but useful if you have a way of uncursing them).
The most dangerous enemies in early game are kittens. Don't try to run away from them; use a sleep spell if you can. Seriously, you don't want your gravestone to say that you were "killed by a kitten". That's embarrasing. You want a tougher, manlier epitaph, like "choked on a dragon".
Most deaths in the game are caused by ants. These things have a habit of appearing when you're wounded, so keep yourself healed if you can.
Don't forget to #enhance your skills. Martial Arts and Healing are good ones to max out. You're almost as good barehanded as with weapons, but with practice your unarmed attacks can become your best offense, so it's good to fight without weapons.
Pets will avoid walking over cursed objects. If they willingly stand on a pile of objects then you know for absolute certain that nothing in that pile is cursed. If the pet is forced to walk over a cursed object then the pet will wimper.

Thank you, Barefoot_Monkey!
I got caught in a bear trap, what should I do now? I can't move and I don't know how to disarm/destroy the trap.

You will escape after several turns of trying.
ADDED: Also, biggest hint: You have time to think out your moves, nothing happens unless you do something. Also, get used to dying, it will happen quite a bit.

I forgot to say this earlier, but that thing I mentioned about eating meat applies only to monks. Other classes can (and should) eat meat freely. You can get special powers by eating certain corpses.
When you are in serious trouble you can pray to your god for help by typing #pray. Gods like it if you pray occasionally, but quickly get irritated if you pray too often, so save praying for emergencies.

Monk isn't very newbie friendly class to begin with. Namely because of the penalties you get from wearing body armor and eating corpses. Easiest classes to begin with are valkyrie and barbarian, first one gets good starting AC and HP, latter one starts with the invaluable poison resistance. Personally I'd recommend playing around with every class so you'll learn to how to cope with various situations, lack of food, proper weapons, resistances, how and when to use ranged weapons, etc.
Keep your pet with you, he's a very strong fighter and can be used to identify whether an item is cursed or not. He will follow you through staircases if he's standing next to you when you go up/down.
Never ever eat a corpse unless you saw what killed it. Never be burdened or worse unless you absolutely have to and know there are no monsters around. Being burdened means you move slower and attack less frequently than your enemies, this is very bad.
Speed is of essence, but you are in no rush. You don't have to go down as quickly as possible, you don't have to kill every monster you see. There is no shame in running from a fight. Going too deep without proper armor and/or weapons is the easiest way to get killed.
Opening inventory wastes no actions, if you are in trouble, check your inventory. Maybe there are unread scrolls, or potions? One of those scrolls could be teleportation. One of those potions could be healing.
And most importantly, when you die, stop for a second and think why you died. And trust me, you will die a lot. Did you have to fight that monster? Did you have something in your inventory you could've used? Did you eat poisonous corpse, zapped wrong kind of a wand? Winners never quit, quitters never win. Learn by dying.

If you kill a lichen, grab the corpse and keep it for emergencies. Lichen never rots. Other corpses should be eaten while still fresh, as they rot soon and can give you deadly food poisoning.
Lizard corpses are very useful mid-to-late game. Save every one you find. Like lichens, they never rot. Eating a lizard cures you of turning-to-stone.

No problem, thanks anyway. I'll give a look at Ancients Domains of Mystery.
But what should I eat? For what I know, or I eat corpses, or I buy food from a seller, but that's rare to find.
But first I need to offer sacrifices for the god, or no?
Thanks, this will probably save me from a stupid death.
How I learn that?
Speaking of stones, I found a big stone and when I touched it, I discovered that it was a mimic, I had to kill it, but was a tough fight. I wanna know if these guys are common ennemies. I hope not.
Thanks it worked. Some grid bugs tried to kill me in the process, but no big deal.
I didn't chose Monk, I just selected random in the options. I've heard good things about the valkyrie, I'll try this class if/when my monk die.
Wise words, I'll keep this in mind.
Yes, if I learn from my mistakes, I won't do it again (hopefully).
Lichens corpses don't rot? This is great!
Just a few monsters have the ability to paralyze the character, right? My friend told me to stay away from a flying eye.

My level 7 female monk died. :( I saw a werewolf and when I approached to it, it invoked help (2 white wolfs), I got surrounded and they started biting me. I hadn't any healing potions, so I tried to use every scroll I had, one of the scrolls invoked more monsters to kill me (see attached screenshot). When I asked to identify my possessions, I discovered that I had a cursed teleportation book.
I think now I'll start with the valkyrie, maybe. What character you guys think I should choose?

Hey, ever read The Lord Of The Rings? Monsters in Nethack can't attack you if you write the word "Elbereth" on the floor. Sorry I didn't think of mentioning that earlier.
Get Slashem (a fork of Nethack with some nice enhancements) and try a doppelganger wizard. I find that fun.

My tip: Read the unofficial Usenet group, rec.games.roguelike.nethack. Be careful though, it's NSFW because of the porn spam (which practically all Usenet and Google groups sadly suffer from).
Ignoring the spam, other players like to post their progress in games, exchange tips and post their ascensions (wins) and deaths. If you go up a level (to rec.games.roguelike), there are Usenet groups for other Roguelike games like Rogue, Ancient Domains of Mystery (ADOM) and Angband.