Pretty easy actually. Basically create a new thread in the uploads section with a nice clearly descriptive title in the appropriate sub section (nintendo,playstation,etc)
Upload your rom/iso to a file hosting site - I personally like using MEGA. And post in your thread. Zipping with a password is a good idea as can lead to your upload lasting longer. Some description of the game, file size and maybe a nice little pic helps too!!
Don't forget to code your links with code tags just type in [code]your game link from file host here[/code] like so.
You can have a look in the uploads section and see some posts to get a better idea as well. If you make a mistake don't worry - a staff memeber will edit your post and let you know what was wrong. Have fun.