Well, aside from the Welcoming Plaza which you probably get the idea of well enough, there's also:
Site Questions & Tech Support, where you can make a topic and ask a question if you're not sure about something to do with the forums. You can also check out the FAQ if you need answers
Testing Forum, if you want to try something out for a topic without everypony seeing
Show Discussion... self explanatory =p
Sugarcube Corner, where you can talk about things not directly linked to the show, such as merchandise, meetups, and so on
Equestria Girls, for discussion about... well, y'know, Equestria Girls... =p
Octavia's Hall, which may interest you if you have an interest in fanworks and all that arty stuff =)
Roleplay World... never checked it out much myself, but, if you like roleplays, there it is
Everfree Forest, for all non-MLP discussions =)
Cloudsdale Colosseum, where there's games and random stuff- I love random stuff...
Life Advice, which you can access once you're muffin rank.
Those are the main things Um, was any of that of any use to you? If you wish to ask me to clarify about something or just to ask about something else, feel free
Oh, there haven't been any episodes since then, so you haven't missed anything Season 5 will be starting in less than a month, though; have you seen the trailers for it yet?