The first step of learning a fighting game is to have total control of your character. If you want to perform a particular movement or attack, practice it until you are able to get it 30 times out of 30. It is absolutely vital to minimize execution error, so that you can focus on fighting the opponent instead of your controls.
After learning how to control your character, learn how to control the opponent. The opponent will want to use his own tools against you, and if you can discourage him from using those tools, you have less to worry about during the match.
For example, suppose your opponent knocks you down. You perform a wake-up attack, but then the opponent blocks your move, then punishes your recovery with a 40% combo. Now, if you get knocked down again, would you want to perform the wake-up? Probably not.
Similarly, after being knocked out of the air for the last 3 times you attempted to jump in, you tend to decide that jumping in a fourth time is a bad idea. Now your opponent doesn't have to worry about your jumping for a bit because he knows you don't want to be anti-aired anymore.
This is how control is established. Make the opponent think that their tool is a mistake by punishing them for using it. You have to learn all of the opponent's tools in order to do this, though, so you'll have to spend a lot of time in training mode and practice matches to find them out for each character.
The next way to control the opponent is to understand how to threaten your opponent at certain ranges. This is done through whiff-punishing at medium ranges, zoning at medium/far ranges, poking out to catch the opponent when they don't block (such as when they are walking or dashing) at medium to close ranges, pressuring with frame-traps and blockstrings at close range, and mixing up with lows/overheads at close range.
You should also be able to confirm off of successful hits in all ranges into a full combo for guaranteed damage. To practice confirms, you can set the dummy to random block in training mode. Then, perform your pressure string. If you see the dummy block, continue pressuring. If you see the dummy get hit, input your combo. Do this until you can confirm your combo on hit 30 times out of 30, because if you input your full combo without confirming the hit, your opponent has the opportunity to punish with a full combo of his own.
The most important part of learning fighting games, however, is to stay motivated. If you still lose to players, ask yourself why you lost that match. Then, figure out how you can prevent it from happening next time. Utilize training mode's record feature to put yourself in the same situation and practice countering it until you find a consistent solution. Fighting games require a ton of commitment, but if you have the right mindset, you can be successful.
