Learn the groundstrokes and serving properly. You can fukk around and kind of play tennis okay, but nothing substitutes for technique. I played tennis for 10 years before learning how to serve properly. Technique technique technique. Watch court-level videos of pros on youtube.
the serve is usually the last to come and the first to go
i started when i was 4 and my serve was never bad but obviously theres only so much you can do until you get taller
strangely enough after my 5 year layoff my serve was the same but my groundstrokes were ass. i had a chuck knoblauch type situation going where the shyt was just broken. idk if it was substance abuse or medications or what but it just wasnt there and has never fully consistently come back. even stranger is that the first few times i stepped onto the court it was like i never left and shortly after it went to the trash bin. with that said i have only lost 2 matches in the past 3 years

but with tennis you hold yourself to your own standards so even if i win im just like

i cant hit that running snap forehand down the line like i used to