Motocross cycling has been a very flexible sport ever since its inception, riders have raced and done tricks with the classic BMX. As the decades have passed the BMX bike has been improved upon in many different ways, such as the categorization of different types of the bike. Innovation has lead to the flatland, park/street/dirt-jump, classic and the cruiser which is a different bike by itself. However they all have one thing that ties them together flexibility with a one break one gear system which creates a bike that can be used for many different situations.
You could be riding on a half pipe in the park or jumping large dirt hills with a BMX but using one that's optimized for the riding you'll partake in. Once you've decided if you're going to use your bike for either racing or tricks you should look out for the length. Most bikes are designated by the wheel in inches and the most common one is the 20 inch bike. Some bikes go up to 29 inches but that's a cruiser BMX at that point and it mostly serves as a comfortable commute bike. And not only should you look to the diameter of the wheels but also the length of the fork and the main shaft since they play a vital role in the bike's flexibility.
If you're building your own bike this is even more important since you have to find compatible parts. However there are many different tubes and bike forks for sale in the BMX market so choosing the right one is made simple. But to actually assemble the bike you're going to need a bit of know-how for where all the parts go and how they're assembled. And that's simple enough, you can find many different diagrams for the assembly of the bike on the internet.
For most people building a bike from scratch is a rather tedious affair so they go for the pre-built bikes that most companies offer, and there is a plethora of them. You can decide on what kind of bike you like and find it by looking through the well-known brands which have manufactured a variety of different BMX bikes. But even then you should look into tuning your bike's feel to suit you by looking through all of the different sizes of frames, handle bars and bike forks for sale which can change the bike's feel completely.