A few top tips:
Donate the first of each type of bug/fish/sea creature you get to Blathers in the museum, that helps you keep on top of them. Ditto fossils (as your museum fills up you start to get duplicates, which help you make money).
Save up your valuable bugs and fish (e.g. dorado, pike, any big fish apart from the sea bass, barred knifejaws, red snappers, birdwing butterflies, emperor butterflies etc etc, although most of those are summer-only) for Flick and CJ - that's how I paid off my house 
Achieving 5 stars isn't that important, but achieving 3 is because that's when you get the guitar-playing dog guy (K.K. something) to give weekly concerts, which help you get music to play around your island on stereos.
Make a habit of hitting your rocks daily. Dig two holes in the ground around the rock like this.... (H = hole, R = rock, dots are only there to stop the letters jumping to the end of the row)
..........H
.......H...RRRRRR
.............RRRRRR
.............RRRRRR
...and hit it with your shovel as fast as you can. No idea if that format will work on mobile!! Your shovel won't ever break in the middle of hitting a rock, but it might afterwards, so it's a good idea to always have a spare one with you. One rock per day will be a money rock, and you can get 100, 200, 300/400/500 (two of those options, can't remember which), 1,000, 2,000, 4,000 and 8,000 bells. NB: MAKE SURE YOU HAVEN'T EATEN ANY FRUIT BEFORE HITTING THE ROCK, BECAUSE IT WILL BREAK AND REAPPEAR SOMEWHERE ELSE THE NEXT DAY.
I've exhausted my top-tips, if I think of any more I'll come back 