As has already been said buy the best you can afford from a brand named brush. Even if you get a "good" cheap one, that works reasonably well, they are usually so poorly made that you will be fighting it to get it to work consistently, and won't be confident whether it's your inexperience or the brush that's the issue, and it will really affect your learning. Also the parts won't hold up, and nozzles and needles will be soft and easily damaged. It may seem that cheap is the best option, especially when you don't want to spend out a lot when you aren't sure if you'll like it, but it is a false economy. You will be spending more on replacement parts, and will probably upgrade and spend more anyway. And buying cheap is more likely to cause frustration and stop you enjoying learning to ABing anyway. Although don't get me wrong, there are times you may get frustrated anyway, but that's what we're here for Lol.
Have a look at some of the options and before you buy any equipment, ask the forum's opinion. Everyone has their personal favourites so there may be a bit of bias toward one brush or another, but you won't get steered in the wrong direction, and make sure you aren't wasting money on something that might not be ideal to what you need.
If you could give more info about the type of painting you would like to do, it will help people guide you towards a suitable type, whether gravity or siphon fed, and a good nozzle size. Then we will be able to advise on what paint might suit. If you haven't got a compressor yet, then again ask for opinions, but look at something that has a storage tank and uses oil. One without a tank will pulse, and is no good, and will run constantly, and without oil will overheat and stop working.