Firstly, welcome to Minecraft. :)
After logging into Minecraft there's Singleplayer, Multiplayer, Languages, Options and Quit Game. Press Multiplayer and there'll be the page where everything will be. Head over to the forums to find a desired server (find a minigame server as those would usually contain many players and work well) and get the ip of the server. Some server ip will look like a bunch of numbers with fullstops inbetween, while others will be words with fullstops.
Click on Direct Connect and paste the ip in and join server!
The ip pasted in Direct Connect will be there until you manually backspace it.
Use Add Server if you want to keep the server in the list (which will be seen right after you click Multiplayer) and you can give it a name.
Hope this helps!