It's easy! Once you have an account, you'll always be @(Whateverusernameyouchoose). Other people form accounts the same way for either themselves, groups, products, etc. (I.e. @CocaCola or something). You look for things you like (example: if you like the voice actor/Luke from Star Wars Mark Hamill) and Follow them (the blue icon on their page). From then on your main hub is not your personal page like it is on Facebook, but the most recent things the pages and people you 'very followed have said, to within a few seconds of posting it.
Think of it as a compact forum where, instead of selecting topics and reading everything that they've posted (which is entirely possible) you are instead getting a window into that forum by seeing the latest posting, and joining the conversation below it (the arrow icon on the left under each posting). If you like something enough that at you want to share it you hit the square recycling symbol (retweet) where it is then linked to your page. The heart is self-explanatory.
