First, have you completed all of Isabelle's "chores" (I think you have to ask her for "advice" or something like that). She should set you up with some tools.
Have you unlocked the Island yet?
You should try to do that. The Island is particularly good for making money (by catching beetles in the evening).
Talk to Isabelle and set your ordinance to Night Owl if you play more in the evening.
Random stuff to do, for starters:
- always sell stuff at Re-Tail (the pink icon on your map); they pay more for your junk
- collect bugs and fish for the museum
- collect fossils (the X shaped marks on the ground, they spawn everyday) for the museum
- befriend your villagers (do errands for them, etc)
- aim to upgrade your house and
main street shops