You get started into UNIX by getting an Intel platform (what ever speed and size you can afford); get a CDROM of a UNIX distro such as Linux or BSD; follow the instructions and GO!
This above approach costs very little money and does not require an Internet connection to download software distributions. From there, you can go to a more network-centric download approach.
The second approach is to go out and get a beginners UNIX admin job; but you will do better if you have much experience with home servers running your own web site or mail site.
As for what you can do, you can do any server side application from web servers, file servers, mail servers, application servers, databases, test controllers, lab gear etc. You can also do graphical desktop applications; but Windows tends to outshine in that area.
Most businesses and people find that UNIX make the best back-end servers and Windows based systems make the best common-user desktop. Both have advantages and disadvantages.
For very sophisticated and complex graphical CAD programs and other visualization (not your typical work processors, spreadsheets, etc.) UNIX makes some very suburb systems!