I started with a box call, and both boxes and slates are easy to use. If you only have one call, make sure it's one that works wet. I put off learing mouth calls for a while because I thought they'd be hard to learn. They are harded than a box or a slate, but not that difficult. I picked up a 3 pack, left them in my Jeep, and practiced on my way to work--Picked it up pretty quick. One problem with boxes and slates is that they require considerable movement. If you don't learn to use a mouth call, consider an "egg" call, also known as a push/pull call (i.e., Knight and Hale's Tom Coffin). You can work these one handed with minimal movement when a gobbler is close enough to see you and you don't want to use a call that might give you away by moving too much.
Good luck.