Make your way to your .minecraft/saves and find the folder that contains you world. Right click it and select 'Send to > Compressed (zipped) folder'. A new file with the same name as the original but with a .zip extension should appear. Now you need to upload that file to a file sharing site, MediaFire being my recommendation. Make an account on MediaFire and upload you project there. Copy the link to the download page. Then, on PlanetMinecraft, go to your account and go to upload new project. At the bottom where it says 'World Download URL', page the link to the download page.