If you are playing a Trooper and you choose Commando over Vanguard by accident you will need to start a completely new character to fix your change. Keep your eyes open for these types of things, this little bit of information could greatly help you in a pinch.Those are the most helpful links that I can think of giving you for helping you find a class. If you're wondering why swtor's official website is not in with those links it is because I feel that the information it gives you isn't very to the point and informative. You should read both of them carefully and even use outside websites/sources for information. The reason you want to make sure you are making the right choice is because once you have made a choice you can't go back.
Here is what you can expect in March and April this year of swtor credits(2018):
I'm not saying it is a bad source of information, all I am saying is that you will most likely be able to find what you're looking for elsewhere and much quicker. As I mentioned before, after you've chosen a class I highly recommend you go to either Torhead or another website that you can look up the specs for that clas and do some indepth research so you know exactly what you want come release day.Trust me on this one, there is nothing worse than leveling up past level 10 only to realize you don't like the specs that are available to you. If you take my advice and want to do research on this before release I highly recommend reading over my class guides as well as using any outside information. Below are a few links that I've used to help me decide which class I wanted to play