Just adding on to what Jeric said, forum sites are just a great way for people to socialize in general, you do not have to have a specific problem or question to join a forum, on this site in particular there is a status update feature which is a great way to share your thoughts with the community or tell people what you are up to, or just have a few shenanigans every now and again.
Then on this site you can create a blog in which case you'll get the chance to share experiences or opinions with everyone else on the site and other people can comment on it and share their feed back on your blog as well, (this is also a good option when you want to share something that may not necessarily be suitable for a topic). Blogs (Unlike the forums) Have no limit to how long or short they have to be, have and idea or something you would like to express to the community? Then simply create a blog to express that idea.
As far as the forums themselves, they are more for discussion about various kinds of topics either pony related or completely non pony related (depending what forum section you choose). You can easily choose what section to post in by simply glancing down at the small description underneath the forum section you are about to click on in the index page of the site and decide what section your topic is best suited for (Just be sure that there isn't a similar topic already started). The Index is essentially a list of all the various forum sections along with their sub sections such as the forum games, fan clubs etc.
Hope that clarified things for you a little, and Welcome to the MLP forums experience!
As others have said you must be at least have 5 posts in order to use a signature.
As a side note, making basic signatures is actually very east to learn to do in photoshop or gimp, and a basic one like mine can be done in little to no time all and with very little artistic ability. The only real requirement is that the image must be 600 by 100 pixels on this site.
As far as learning how to make them, there are a lot of very informative bits of information on the net about the subject, I would suggest going to youtube and typing, "How to make a forum signature." there you will find videos of people using various software such as photoshop, gimp, and MS paint.And remember, different forums have different size requirements so before you go on to tackle the task of a do it yourself signature make sure that your dimensions are set correctly so you don't go to all the work of making one just to get an error that it is to large.
Once you have created something that you are pleased with upload the image to an image hosting site such Imigur and right click the image and click "view image", then from there all you have to do is add the image to your profile so it will appear under your posts, to do so go to Your profile>Edit my profile > signature, from there add your signature to you profile by going to the image button that is located under the emoticon button and copy and past your image URL.
Really the only advantage to making one yourself vs having someone else make you one is that it will feel more personal to you, but of course if you would prefer to just have one made instead of going to all that trouble there is always someone around here that would be more than willing to make you one.
Oblivion's post in Website Uses. was marked as the answer