The shape of the shield you use is a big thing to consider for anyone. Your shields shape will change the way you move, defend, and attack. the way and angle your shield is strapped can change things as well. Take your time and really think it over.
In my opinion the strapped round is the best shield for beginners to learn with. The shield I am referring to is approximately 2′ in diameter and is strapped with one point in your hand and the other on your forearm. The hand grip should be closer to the center of the shield than the edge. This lead to a bit more of your shield being extended from your hand, extending the reach of your defense.
I believe this is the best beginning shield because of its versatility, ease of basic use, and the ability to take the methods and techniques and easily transferring them to other shapes of shields. If a fighter begins with this shield they will have a more shallow learning curve, meaning and easier time, when learning any other type of shield.
The arm motions used are similar to the ones used with other strapped shields. The angles of your edges are similar to the buckler. Your hand placement and pressure is similar to the way a center gripped shield would be used.
If a fighter starts with other types of shield, yes they may get greater success early on, but that ease leads to tougher growth later. If your goal is to become the best you can than every set back is an issue, anything that can slow you down is a problem.