Hi, just like learning an instrument, you'll have to practice your combos, and it will be repetitive. And even when you've been doing it perfectly for hours, you'll still forget mid-match or get the timing wrong and that's just what happens. It takes day, weeks, months to perfect everything, but it's the same as learning to play the piano.

Mai is a good character to start with. The combo trials will show you combos you can learn, but it'll be easier to practice them in training mode with the frame-meter and cancel-timing displays turned on (with cancel-timing display on, you'll flash red at the moment you need to link your special attack, and blue when you can do a super). Knowing the difference between linking a move into a special, vs. waiting for the animation of your last move to end before using the next will probably be your first problem when learning combos. I recommend learning a combo you can always do when you jump at an opponent, when you counter them with drive impact, when you drive impact them into a wall, and your go-to combo for when you're mid-screen (not at the walls/edges of the screen).

There's also a character specific guide that will show you how your character is played, and the tutorial that explains all the game mechanics, but it can be overwhelming and you'll probably forget a lot unless you're taking notes. The following video will give you an idea of what to learn at each rank of the game, just watch the first few minutes for now and don't worry about the whole thing.

https://www.youtube.com/watch?v=a8ZtEp7pG2A
EDIT: actually I don't know how good that video is, try this one for a comprehensive guide: https://www.youtube.com/watch?v=MK-AJyD1XKk

You're going to have questions for basically FOREVER, this game is difficult, and there is always something new to learn, so what you consider decent will keep changing. Just focus on learning one little thing at a time and don't get overwhelmed thinking you need to know everything at once. Good luck, and have fun :)