Welcome aboard!
There are many very good GUI's out there but I might recommend using stock until you get all the concepts of manual targeting down before trying different GUI's.
OLC's GUI's are good as well as the others that use a version of it (ACM/ACM Reloaded, Manos). The in scope wiz wheel helps figuring range, AOB, and speed a lot easier although there are other ways to find out that info as well.
I am currently using Hitman's GUI 1.0 which is very good and does away with the in-scope wiz wheel. Alot of the information is figured out by visual cues and, surprisingly, I am getting better at figuring things out by eye.
However, all of the above is useless if you don't understand some of the principles and concepts of manual targeting.
The best thing to do when choosing among all of the very good GUI's is to try them all out and see what you like and dislike about them and choose the best fit for your style and experience.
I've found that they all have things I like about them and things I don't like. One GUI will always have something you wish was in another GUI.