Whatever site you use will usually have a tutorial. Play texas hold em is probably the most popular. Learn the best hand to have firstly from high card , to a pair etc all the way up to royal flush before you worry about betting big. Start out playing 10p games for fun until you find your game.
Learn by watching poker tournaments on TV. If you have Sky then there is almost always a channel showing poker, in fact channel 846 is Sky's own poker dedicated channel.
Once you start playing don't even think about putting money into an account for a good while, I actually waited a year before i started playing for money, play the "play money " tables, Party Poker is a good site to start on.
you will play texas holdem probably. you have two cards in your hand. there is some betting, then 3 cards come, then betting, then another, then betting, then another, then betting. you have to try to make the best 5 card hand using none, one, or both of the cards in your hand. you have to try to work out if your cards are better than other people's there, if it is, you bet or raise.
hand rankings easily searchable on web.
There are hundreds of online poker tutorials out there... just be careful not to get sucked in too deep (financially) too early. A good starting point for information on gaming strategies is www.win-ipedia.org