The Alien Invasion expansion, as well as the free play version, both start you out on ICC "Newbie" Island. It's a good place to learn the tricks, and there's always lots of other newbies to learn from and with. For a detailed walk about the island with strategies & tips, visit here
If you have the AI expansion, you will want to stay here a little longer. Level 15 will let you collect your next Alien level, and save up the XP for your next (though you can't "ding" (gain) that level till you reach 25).