Hi rozzie! Welcome to the forum.
Carl has written instructions for posting and uploading items to the Swap Shop at the bottom of the
Swap Shop Instructions and Guidelines page. Basically, when you're happy with your finished product, you click on the "Share" button (in Edit Town for lots and households, Create-a-Household for sims and pets, Create-a-Style for clothing and build/buy mode objects) and the item shows up in the Exports folder in your The Sims 3 folder and under Exports in the launcher. Now, you create your thread here in the Swap Shop, under the appropriate heading, click on Additional Options bottom left of your post and attach the desired file from your The Sims 3 Exports folder. Now click on post and wait, it can take a few moments.
Feel free to PM me if you have further questions.