Don't put too much on it at first. Play around, fuck the system up, reinstall a couple times. Great learning experience.
Agreed. The first time you install it, be completely ready to install it again. Play with it, and when you get yourself into a corner re-install it.
Also, read the instructions. You boot off Slackware Disk 1, then, if you follow the instructions, you should have no problem setting up your partitions and performing a full install. Most people who ask me questions about how to do it do so because they don't read. But really, the prompts all tell you what to do, and it should be fairly common sense.
Also, the Linux slogan is "./configure, make, make install". Those three commands are used to compile 99% of software. You won't have to know that until after you've installed everything, but it's still good to know.