15 minutes a day is not a lot, especially when playing games. 10|0 games by themselves can take a maximum of nearly 20 minutes to finish. Blitz is much worse to play, as you don't have much time to think.
The minimum time control I would recommend is 15|10, because you're able to think a lot longer. The longer you think, the less likely you will blunder.
Before you make a move, look ahead as much as you can. Also think about what your opponent is thinking of doing. "If I do this, he can play (insert thing), then I can play this, and he can play (insert thing)." You get the point.
When it comes to tactics, particularly practice hanging piece tactics and mate in 3 (and less) tactics.
I see that you don't have a Premium chess.com account. Because of that, you should use Lichess. If you don't have an account, create one. After that, do the following:
1) Hover over the section called "PUZZLES".
2) Click "Puzzle Themes".
3) Find the following tactical themes: Mate in 1, Mate in 2, Mate in 3, and Hanging Pieces.
4) When you click on a tactical theme, it groups them from "Easiest" to "Hardest". Start with the easiest section and then work your way to the hardest section.
Note: Doing random tactics will help, but your progress will be slower. The reason is because it will take you longer to familiarize yourself with tactical themes.
Hope that helps.
My aim in chess is to defeat anyone below 1000 with ease and confidence. But as of now, I am way behind a 1000 rated player. Why is that so? How can I plan to achieve my goal with 15 minutes a day dedicated to chess?