Navigate to your .minecraft folder by typing %appdata% in either File Explorer or the search field in your task bar (if you're on Windows), go into your "saves" folder, and find the folder for your map. Right-click on the map and click "send to -> compressed (zipped) folder." Once you have the .zip file, find a file hosting site online such as Mediafire, Dropbox, or Google Drive. Make an account (if you don't already have one) and upload the .zip file to the site. Once it's uploaded, copy the link for the file and paste it in your Minecraft Forums thread.
Always make sure, once the thread is posted, to log out of your account and make sure that the link works correctly and that it allows you to download the file again. Quite a few users accidentally set their file to be "private" when they're uploading it, which doesn't allow anyone besides you (when you're logged in) to access it. It's quite frustrating for users to find a cool-looking custom map and not have permission to download it.