If you could only have one call as a beginner, I would recommend a "push pin" call as pictured below. They are easy to use, require little movement, will make all basic calls and you can control the volume fairly well. The box call is the oldest and most widely used call, but they are fairly loud for all purpose calling and present problems when it rains.
You should also get a simple 2 reed mouth call. The entire turkey's vocabulary can be difficult to master on a mouth call, but for now you only need to learn a simple cluck. This is to get a gobbler to stick his head up for your shot when he is in full strut and it does not require the use of your hands, allowing you to keep them on your shotgun.
Lastly, a crow call is handy to try and locate turkeys by getting them to shock gobble in the daytime. They are inexpensive and easy to use. I have pictured a Knight & Hale call because you should be able to find it fairly easily at your local WalMart, Gander Mountain, Dick's or Dunham's sporting goods.