Seriously, since Java has a C/C++ like syntax, it shouldn't be too hard for you to read the code and look at this... You probably should follow the initial instructions to get the codebase with Git (and if you are not familar with Git, do a basic tutorial from say Github helps pages so you understand the basics commands i.e. clone, checkout), then firstly compile for your desktop platform the latest build... This will allow you to get the basics before you try sorting out the PSP cross-compilation environment... but copious documentation of this is available on our Wiki, mainly linked from Development Central:
http://wiki.scummvm.org/index.php/Main_Page
Rather than asking any questions on the forums, I'd suggest reading the documentation on the wiki, trying the basics and if you get really stuck, then ask in our development/user support IRC channel (#scummvm at freenode.net):
http://wiki.scummvm.org/index.php/IRC_Channel
You will usually get a good answer there, but consider that most devs are European, so best times to ask are EU morning or evening.