If you can, try shooting different models and calibers before you purchase your centerfire handgun. I would also look into taking the NRA Basic Pistol Course before you purchase a centerfire handgun. You will be taught good shooting form and firearm safety along with things to consider when purchasing a handgun.
As far as Glock, some people love them and others hate them. The same can be said of any manufacturer. Find one that fits your hand well and works well with your natural point of aim.
Of the three most popular semi-auto cartridges (9x19, .40 S&W, .45 ACP), the .40 S&W has the snappiest recoil and loudest muzzle blast. The .45 ACP is more pleasant to shoot than the .40 S&W and is considered by many to be a better stopper. Several departments have moved from the .40 S&W to the .45 ACP. If you get into reloading, reloading the .45 ACP is easier than the .40 S&W.
Stay away from .45 GAP. Also be aware there are several different 9mm that are different cartridges. The 9x19 is the most common in the U.S. and is also know as 9mm Luger and 9mm Parabellum.
9x19 = 9mm Luger = 9mm Parabellum