In order to upload you'll need to get tracker account if you don't already have one. If you need one, goto
http://www.datorrents.com and click the Signup link in the upper right hand corner.
As for how to upload you'll need to create a torrent for the files you want to upload. The exact details of which depend somewhat the bittorrent software you're using. If you'll post the name of the software we can probably give you instructions for it.
After you've created the torrent you'll want to upload it to the tracker by logging in and clicking the Upload link at the top of the DaTorrents main page.
Once you're on the upload page give your torrent a title then click the browse button next to the box label torrent file. Find the torrent file you created and click open. You can ignore the part about an NFO file. Once your finish write description of what you're uploading then click upload.
Once you have it on the tracker you'll need to seed it. You can do this by just opening the torrent file you created like you would any other torrent file. Once you open the torrent you'll likely be asked were you want to save it, you should point it to the location of the files you want to upload. The software will check and see the files are already there and begin seeding. It may take a little while for someone to come and download from you.