Surfthosewaves, at your age, height and weight a short board will be ideal to start on, 6ft to 6ft 4 inches a good width say 20 inches or a little more and thicker than 2 1/2 inches should be fine, within those dimensions you should be able to get a good learning board that will fill your needs for a couple of years.
Yes you will struggle a bit ,it is unavoidable no matter what board you start on, but at 12 you are at your most adaptable and a board around those lengths most young guys learn on OK .
The longer board suggestion is usually a better option for older fatter and heavier surfers as their starting point and then they shorten down.
Chose a board now and go surfing

, the answers to surfing lie in doing, not thinking too much.
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷