Try to figure out what sort of things are making you lose and prioritize fixing those.
can't be stated too many times. If it's possible for you, record some of your matches, these should be very helpful in figuring out those things; in the middle of the match it can be hard to self reflect and see your errors. If you can't, you can always ask your friends what they think makes them win against you.
some points that might need attention:
- do you know what you should be doing? (basically stuff like "my dair on falco is very good, am I (ab)using it accordigly?", "even though my fsmash on peach is not that good, but if the opportunity arises, do I recognize it?" What are the basic bread and butter combos of your character? What should your gameplan vs the other characters look like ("peach won't die to the side until very high percents, should I try to kill her vertically?")
- are you too obvious in your approaches / other stuff?
If your approach consists of dashing straight towards your opponent and dash attacking every time, it will probably be pretty easy for him to shield grab you (especially if you play against each other a lot

). If you mix it up with jc grabs / shffl'ed aerials out of his grab range / landing on the other side of him / ..., he'll have a much harder time.
- is your tech skill on par with theirs? Are they able to perfectly shffl / waveshine and whatnot, and short hop on characters other then bowser is still hard for you?
other then that you can view tournament videos of your character to see what good players tend to do in what situation, and try to implement this stuff in your play, and/or check the character sub forums for more specific help (including some more information on your play / your usual opponents / ... might result in more detailed information

)