Look at the Education Courses and Degrees. They take time but can boost things (Mathematics can help with ammo conserving for example).
If, like me, you're a free player look at Spin The Wheel. Try to hoard 25 Points from there and buy a Stock Ticker. You can stash your money in the Stock Market and it's (relatively) safe.
Arson requires a Driving Licence (gotten from doing Social Sciences Driver's Ed Course) and a Gasoline Can. Burning down Warehouses gets you cash & XP.
If you are going to fight don't dismiss the Blowpipe as an opener. Poison Damage increases over time.
NOW is the time to buy a Chainsaw. It'll be useful come Hallowe'en.
Hit up Bits 'N' Bob's and buy your daily 100 things there. No beer? Buy either Fruit Cake or Teddy Bears and put them on the Item Market.
That Piggy Bank you got? You can only open it once. Put cash in there you can spare until you can access the Stock Market. Once it's gone it's GONE.
Scour the Map for objects lying around every few days. You might get lucky. I did. That Parachute I found bought my Private Island. The Pilot and Airstrip took longer.
Flying from Level 15 and loading up on Flowers or Plushies to sell on the Item Market is good money. But it takes time. Be patient.
Get a job. Rise through the ranks in your job and get permanent benefits. Top rank in Teaching gives 10% shortening of Education Courses even after you change jobs.
Hold your Level from 15 if you plan on being a Tourist for a while. Just remember when you'll land as after 15 seconds some bugger could mug you meaning you lose money and time. Time in Hospital is time not in the air.
This game is a Slow Burn. It can take years. Be patient.