Party Poker - One of the oldest and most respected poker rooms in the industry. They offer all new players a large sign up bonus where players can claim up to $500 free.
It's easy to get started. The superb software is intuitive to use and offers plenty for all types of players.
The number of tournaments is quite impressive. Each weekend there's a guaranteed 400k up for grabs.
The number of Sit N Go tournaments is enough to keep anyone happy.
If you just want to play non tournament Texas Hold Em then you'll be pleased. The comp program also ensures that you are well rewarded for playing here.
They also have a guide that teaches you how to play, all versions of poker from Texas Hold Em, to Omaha High, and 7 Card Stud.
Below are screenshots of the website, the lobby, and of the actual software.