So, since this is my first ever server, I've got a couple of questions.
1. What's a good OS that has plenty of support (Help manuals.), and can perform a large range of processes.
Most, other than the most esoteric OS's are well supported and documented.
Do you want ease of use (Windows), outstanding security (Open or FreeBSD) or bonzer reliability (Linux)?
2. What's the major difference between a SCSI hdd, versus one that you'll just put in a desktop? I know that there's a physical connection difference, but other than that?
Most people I know use a SCSI in their desktop
For a server, you want SCSI. Most desktops have IDE something or others. They are cheap and good for one or two users or so, but would bog down right quick in a server. IDE's are a good option for backup to the main SCSI.
3.What are more possibilites that you can use a server for besides the very basics, (Websites, Gaming)?
Depends on what you want the server for. Most used are:
WWW - Apache
FTP - vsFTP or ProFTPd
NNTP server - Leaf
SMTP server - exim