Cool that you are giving this career some thought. I had wanted to be in the FBI when I was young, and then in high school had aims to work as a police officer. My parents steered me to college. I did that for several years. Got a B.A., and M.S. but wound up working with criminals as a counselor, then worked in corrections for many years, and realized where I was heading so I started to apply for agencies. I applied for one that I worked for twice, and word was, that was typical. Acceptance on the second application. Although I left the big agency and began working at a smaller agency. The difference has been enjoyable, and I feel I've found a place for my career.
I made sure to build skills that would help me in a law enforcement career, I'm not sure its necessary as I know one cadet in my class had sold video games before hiring on, although I don't know their outside interests. Our field is made up of so many people, former navy seals, math teachers, former sports stars, musicians, third generation officers, etc. I'd always liked shooting and such so that was easy. I had verbal skills from my former jobs. A lot of policing is customer service, and talking to people. Couple years of krav maga rounded out physical skills. I also had developed some professional interests and became active in dealing with them at work, as well as gained membership in related professional organizations.
I attended the academy of a large department here in Texas. It is held to be one of the best in the nation. I do feel that the training was excellent. It was detailed, expansive, thorough, and grueling. I recall marines in my cadet class getting peeved with all the PT, as we not only had gym sessions for fitness, but we had a vast amount of smoke sessions for various foul ups. It was tough, but it teaches you how far you can go, and that you can get through whatever. If you like action and you like school then you will like the academy.
As far as first year(s) you are going to see a lot of things you may not be used to. Domestic violence, child abuse, sexual assault victims, psychos, gang members, juveniles in need of serious supervision. Transients that can stink up a cell block by taking off their socks, drugs coming out of places you don't normally put things. You might get hurt, the people you know might get hurt. A feeling of jadedness, but also an awareness of the part of life most don't really like to dwell in. As well as the people who appreciate what you do. The victims of assault, of robbery, and of theft who you helped when they really needed someone most. You're going to calm down hysterical people who just had motor vehicle collisions, who are feeling suicidal, or who might be losing touch with reality. You might get the smile on the child's face who was so happy to meet a cop, and you might give a homeless person a bottled water or bag of chips or whatever and got a smile from that. You're going to work with great people and I hope you gain lasting friendships. It might be hard to get out of cop mode, but keep your hobbies, keep your friends, kindle that non cop life. You're always a cop, always on duty, but don't forget yourself, your family and your community, be there for them as a person as well as an officer.
As for academies. The area I'm in has several options. The larger agencies have their own academies. Some are held more frequently than others. If you have a Peace Officer license and have policing experience, there usually are shorter academy or "modified" options. There are a few private outfits which provide training, and smaller departments might send an applicant to those. Our local collages also have degree plans, or professional vocational training for law enforcement. You spend a few years, and get a degree and a license. The license isn't any good until you are picked up or "commissioned" by a law enforcement agency in the state. Texas has a state specific licensing agency, the Texas Commission on Law Enforcement; TCOLE.
You might see if there are any free licensing exams available for your state. A university in my area has practice exams. I took them to help study and prepare even before hiring on. Also gives you an idea of the material you will study, and can help you get a feel for wanting to do this or not.
Before the academy and everything there might also be exam prep books for you. I had one years ago which featured a little self eval, and had a score associated with your fit for the job. I thought that was neat, although it just confirmed what I wanted to do.
Best of luck to everyone.