Hi, Epiclove.
If you want to upload a sim, it's actually very simple.
Go to the relevant section in the forum. For Sim 2, it goes in the
Custom Sims section. For Sims 3, go to the
Pudding Exchange.
Click "New Thread". Type up your post and add your pictures. A couple things to remember: please make sure you either add a credit list including links to any custom content, or, at the very list, add a Clean Installer Screen Shot. Also, please make sure you aren't breaking any creator's TOU by sharing any content. If you do use content a creator doesn't want to be shared, remove it from the sim and link to it instead.
Now, onto the sim upload part.
At the bottom, left you'll see "Additional Options" click on the plus sign and you should see "Attachments". You can attach ONE image and a file. Now, this step is important. If you want a forum view thumbnail image, select your image FIRST, then click "more attachments" and select your zip/rar file.
Click on "Post" and you're done!