have you ever ridden a bmx? or a street trick bike or a mountain bike? there are 3 sizes: mod: 20 inch ( sometimes 19 the backwheel), same as BMX, 24 inch ( some trick bike between stock and mod) and stock, 26 inch ones. Basically they arfe the same size, only the wheels smaller or bigger, the chase and the bar are roughly at the same position, so it doesnt really matter how tall are you. I am 185 cm and 65 kg and i ride a mod, and it is good for learning, altough i will change to 24 inch ones. At first, buy a bike on your experiences. if you are familiar with the small wheel, mod, if the bigger ones, stock. Try to buy the best you could. After that, when you will start to learn trials, you will see that you like that size or not. You could change your bike after that.
So at first try to find a good value bike. It does not really matter, what brand is, the most important thing is how good condition is it. After you got, find a spot ( preferebly a quiet spot, you wont get ashamed by any body else or its depenging on you, but if there are other riders, its better) then practice as much as you can,