Login information should have been sent to your email or whatever so you just need to type your 'username' and 'password' into the box thingies at the top of the screen and you're away... you should be sent to the members area, and if you go into the 'hosting' section and then onto the 'wizard' thing you can start to build it, there's two options to start off with, if you want to customise your forum completely then to on the second on cos the first one uses predesigned themes.
Like your handle by the way 'Manu', I upon occasion go by the handle 'Oriel' ;)
If you got your login in info then log in. Decide which editor you want to use. One is a combination wzwig and HTML the other is called the wizard. Look and play around with both to see which one you like tthe best.