I know shotgun is even scarier than handgun somehow, but if you have no experience with guns or aiming, you need to do some very careful research on where to aim your gun if using a handgun of 9mm caliber. You will also likely need to practice at the shooting range. 9mm refers to the "strength" of the bullet, and JHP means Jacketed Hollow Point. There are two main types of bullets, JHP and FMJ (Fullmetal Jacket). FMJ is mostly used for practice at the range, while JHP is used for self-defense or by police. FMJ does not expand upon impact, whereas JHP does. Therefore you want JHP as it will cause the most damage, as opposed to FMJ which has more likelihood of passing all the way through your skull and not damaging enough. It is perfectly normal to buy JHP bullets at the gun store, so don't worry about looking suspicious.
If you are set on using a handgun, practice aiming at the range, and practice where to aim in a mirror. See the guns megathread in the suicide resource compilation for some helpful info on where to aim. It's a long thread and can get rather dense but you don't have to read all of it, the first 5-10 pages will tell you most of what you need to know.
https://sanctioned-suicide.net/threads/guns-firearms-megathread.9042/
9mm JHP will probably work, but there have been cases where the person survived. I don't want to scare you, but these are important things to take into consideration. If you want more peace of mind, I would go with a .357 or .44 caliber gun (generally, the higher the number, the "stronger" the bullet). There is more info about these as well in the guns megathread. As for my personal advice, I would still recommend a shotgun (12g with #00 buckshot). Don't ever aim at temple or under your chin, whether with handgun or shotgun.
Good luck my friend. I hope you find peace whether in this life or on the other side.