Hello I am 14 years old and I have absolutely zero experience with dirt bikes.
I was wondering what dirt bike I should get,
I am 6', 138lbs, and my parents don't really care what kind of dirt bike I get.
I am mostly going to use a dirt bike for going around on our property which has some bumps/hills and going to school.
I was thinking after reading somethings that I should get a 125cc 4-stroke racing bike.
Also my max budget would probably be around $800 (depends what I can get out of my parents).
The "going to school" part really changes things. I recommend eliminating that goal. A street legal dirt bike is usually a poor blend of two different types of bikes and/or a lot more expensive.
At 6 feet tall (and only 14!) you need a full size bike. A four stroke in the 125 cc range would be a small frame and wouldn't be suitable for you.
Your $800 budget is going to be tough. You can certainly find some good bikes for that, but I would expect it to be a 125cc two stroke or a 250cc two stroke.
You will also need some riding gear, which will cost a few hundred more. Do NOT get on a bike without, as a minimum, helmet and boots. The need for a helmet should be obvious. You need good dirt bike boots because it is highly likely you will kick a rock or similar at 30 MPH and if you don't have decent boots your feet can be really hurt!
I also highly recommend goggles, chest protector and gloves. These are ESSENTIAL if there is another bike riding with you!
Some times you can find a "package deal" where you can get a bike and riding gear. I bought my first bike (YZ 125), along with boots, helmet and chest protector for about what you are talking about, adjusted for inflation :-) There are a lot of people that are selling the bike because they are getting out of the hobby and they don't need the gear anymore. With a little luck you will be the same size!