ghaib - there is a themes section of this forum that may be of more use in regards to theme customization, but I'll take a stab at your questions above:
Is there any guide for creating a custom front page?
The "front page" (main site theme) uses the one in the "home" directory by default. I'd suggest copying that and tweaking it rather than trying to do a theme from scratch, at least until you're more familiar with things.
Is there an admin section, where you can see all blogs?
It should have given you your admin login when you first got it up and running. If you login with that you will see a "Site Admin" section. I'm guessing this is what you were asking, but I'm not sure how you could have gotten it installed without seeing that.
I uploaded new themes, they don't show up... How do you set a custom default theme?
Themes are in the theme directory. In the admin pages you will see options for managing them, although these can sometimes be a little quirky. New blogs will use the theme in the "default" directory by default, so for a quick and dirty "default custom theme" you can just edit that one.