There are a number of editing tools out there, it just depends on how in depth you want to get.
In terms of editing/building programs, the most straight forward is Compulsion's PESfan editor (
http://purplehaze.eclipse.co.uk/editor)--however, as far as I know, it is only available for ps2. Another popular program is AFS Explorer (
http://www.pesboards.com/showthread.php?t=87), its a little more complicated than PESfan but you can do a lot more with it. There are other tools out there as well, spend some time googling and you'll find any number of them. Try a few out and see how they work, then choose the one you like best. Remember to make sure you editor works on the platform you want.
If you want to do your own kits, boots, balls, banners, emblems, etc. you first need a design program like Gimp or photoshop (I use gimp b/c I don't have the money for photoshop). Inserting them can be done in a number of ways, if you google for tutorials you will find a few decent ones. Kitserver (
http://www.pesboards.com/showthread.php?t=71) is the easiest way to insert kits.
Its really about trial and error, there is no PES editing school, so you just have to jump in with both feet. Get you file onto your computer and start playing with it. Spend some time googling, you'll find all sorts of programs and info. Also, check out
http://forums.evo-web.co.uk/forumdisplay.php?f=216, the threads there have all sorts of valuable info but don't clog their boards with tons of threads pleading for help because chances are no one has the time to hold your hand. Search the all the forums you can find, google, take the initiative. Also remember that even with 2008's limited in-game editor, there is still a lot you can do. I've been having maxdrive issues and while I am waiting for my new one I did all the EPL and Bundasliga kits with the in-game editor (check out
http://www.sakax2.com/news/), not to mention all the transfers using
http://www.footballdatabase.com/index.php--an invaluable resource. I realize that I have provided only the bare-bones, hopefully I don't sound like I'm scolding--but no one has the time to hold your hand, however, regardless of what some pricks might say it isn't to much to ask for some info to get started

. Hope this helps. I can answer some more specific questions but I am not, for instance, going to teach a class on hex-editing. I'll try to point you in the right direction though...