There are a couple of different ways you can join a game on Replay Poker. The easiest way to just jump right in is using the "Quick Play" button at the top of the Dashboard page. This seats you at the first available table where you have enough chips to play.
If you want to be a bit more selective with the table that you join, you can browse the Ring Game Lobby instead of using "Quick Play." Using the filters at the top of the page, you can narrow down the lobby list to show available low stake tables that you can join.
When you find a table that looks good, select it in the list, and then join the fun by clicking "Play Now" in the box on the right side of the page.
Both of these options will seat you automatically and you can get to playing poker!