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?

Why don't you make save backups regularly? I know many people call this cheating but I hate starting a new game after investing many hours into a game.

Save scumming (as it's called in Roguelikes) defeats the purpose of having permadeath and ruins the challenge. It makes the game a cakewalk. I wouldn't criticise a person for doing it in general, but any ascension you get by this sort of gameplay is illegitimate.
Edit: Nethack features "Explore mode", which is a legitimate way to cheat death, among other things. If you start a game in Explore mode (as opposed to switching to it later), you'll start with a rather handy Wand of Wishing, which allows you to acquire practically anything in the game. How is this legitimate? Your score isn't recorded at the end of the game, and once entering Explore mode, you are unable to turn it off. It's a good way to learn the ropes and mess around a bit.

Another good variation (for me anyway - I don't want to start a 'version wars' free for all) is Dungeon Crawl Stone Soup, available in tiles or Ascii flavours.
I find the UI more than helpful at getting you into the game...
[url=]https://crawl.develz.org/wordpress/downloads[/url]

I think the best hint is to just play. And die. And learn from your death.
You're not really meant to complete this game. It's about having fun, dying, and then having fun with a different character.

You might find it useful to learn from other's mistakes. See Lessons Learned The Hard Way and Bad Idea in the Nethack wiki.
Discovering interesting and embarrassing ways to die is part of the "fun" though.

Thanks everyone for the tips, I really appreciated it. ;)
I marked Barefoot_Monkey with solution, because he answered most of my questions, and was who posted more replies.
Congratulations, Barefoot_Monkey, your rep received a boost!

Nethack: a strange game, the only winning move is not to play.

Unless you make backup of the saves, or use "Explore mode", there'll be always a risk of dying. The death is permanent, there's no way back, and this is what makes NetHack so challenging.

damnit. Now I got sucked into NetHack again (or SlashEM, to be precise).

Yay! Thanks :)