My department issues the Glock 21, 22, or 23, depending on hand size. If you have small hands, the 23 would likely be the best choice. The other benefit is that it is a bit smaller than the others, and off duty carry would be easier with it.
I'm not sure what your choices are, but out of the three, I perfer the Glock 21. Something about the recoil being easier for me to control with a 45 caliber, rather than a 40. The glock 21 has a very smooth recoil. I've never been a fan of the 9mm, just my opinion.
Good advice on Practice, Practice, Practice. The better I become at shooting, the more I enjoy it and have more confidence when and if the day comes.
My department supplies ammunition as well, so you may think about your decision based on what they will supply for you to shoot and practice with.
Congratulations and good luck in the academy!