List of things which seem to be the most regretted by more experienced cubers:
1) Not exploring other methods (I would recommend checking out at least Roux, CFOP and ZZ before deciding on one to stick with).
2) Not starting off colour neutral (fully for CFOP, x2 y for ZZ or Roux)
3) Not learning to use inspection effectively (basically just try to plan as much as you can with unlimited inspection).
Other useful advice/stuff:
1) Try to learn fingertricks and find multiple ways to fingertrick each move (such as push, lefty reverse flick and normal righty flick for U moves)
2) links to the point above, but try to move your hands as little as possible to perform each move as this will increase time.
3) lookahead: basically, while you are doing one step, try to plan for the next step (so for CFOP, while doing cross, plan your first F2L pair, while you do your first F2L pair, try to plan the second).
4) try to avoid pauses: this is similar to the above but try slow solving where you never stop turning but don't turn at the fastest you possibly can remember: slow is fluid and fluid is fast.
5) Linking to point 2, try to make all your movements smooth and blend into each other rather than being separate and jerky (see Alex lau and feliks zemdegs for good examples of this).
Anyway, I wish you luck on your cubing journey and remember: you can only get better from here!
