Getting A Pack
To get a pack, visit here for the information you need and where to send it.
When You Have A Pack
You will need to create an XML document to start building your pack. An explanation of this process is available on the XML Guide page. Login to the admin area and then go to the Versions for your pack, then edit the dev version. Here you can paste your XML for your modpack.
You can also navigate to the Files section to upload your mods. You will likely need a copy of Forge from the website http://files.minecraftforge.net/. Read the XML Guide for more, but you will need to use type="forge" for this mod. Start off slow and add more mods once you understand the syntax.
To see your modpack in the launcher you'll need to add your Minecraft username as a tester from the Settings page. Please fill in any other fields such as pack image and description.
Once you've built your modpack you can publish your pack. This will make it visible to anybody on the whitelist (also in the settings page, not your server whitelist). Published packs need to have the correct permissions and donation links so ensure everything looks good before publishing.