Most dairy farms use A I so that they can pick and choose bulls that will compliment their cows so that the (hopefully) ensuing heifer calf will be better than her dam. Improving the genetics. Also, by using A I you can choose from many different bulls and not have everyone bred to the same bull. You don't have to "change out bulls" if you keep the heifers so that they do not get bred back to their own sire...and 2 years would be luck, most heifers will start coming in heat anytime after 12 months, so a new bull would be needed yearly unless the heifers are kept well away from the bull so he can't get to them and get them bred back too young.
That said, many dairies also keep a clean up bull so that cows that don't settle to an AI breeding after 2 or 3 times, they go with a bull so that the cows don't go too long between calves. The ultimate is 12 months, practically it is 12-15 months on many farms if the cow is a high producer, she may not come into her first heat after calving for a little while if she is putting all her energy and body reserves into producing milk.
Yes dairy bulls tend to be more aggressive than beef bulls; some of that is because they are often kept penned in small lots and not used as much for breeding as beef bulls that are allowed to run with the cows, and have a more natural type existence. Beef bulls do tend to be quieter but NO BULL is to ever be considered as a pet. They can turn on you as can any cow that is feeling threatened. Especially one that has a new calf to "protect".
Since you said you are a teenager, I really do think that you need to get some basic raising a calf/steer/cow type books like the Storey publications I mentioned. Get some of the basic info on what it takes to keep an animal, feed, hay, equipment, vet needs/costs. Then after some thorough basics, decide what you want to raise.... and WHY YOU WANT TO RAISE IT. Then worry about some of the other things, like breeding calving and such it that is where you want to go. I think that getting in with a 4-H or FFA group, often through school, would be a very good way to get some practical hands on experience.
The first thing I would do is get hired on with a farm do that for a couple of years then see if u want your own cow. That way u can get payed to learn. Not loose money to learn
Also to be considered along with fencing is handling. Meaning corrals, loading chute, and working chute. Because its inevitable weather you have one, ten, or a hundred your going to have work them. Weather it be loading, giving vaccinations, worming, castrating, assisting in birthing, tagging, or any other vet work. And your going to have a system strong enough to hold them and safely handle them.
When getting into cattle buying you stock is the cheep part when starting from scratch. Its everything else that really costs the most. You can buy a good cow for $1000 but it'll cost 3 times that in fencing.