First: If you just started playing your first thing to do should be to complete some of the first missions. They are easy and you get cash and respect.
After: when you completed some missions and have some cash start with buying ammo cause you will need much. Try to take over some business and rackets to earn some income.
Cash: To earn cash you can take over rackets but you can also try to do some hit contracts and try to do the bonus conditions, it is easy on the first contracts. Another good way of earning money is to rob banks; the bank near your first safehouse is the easiest one. If u going to rob it start with making sure that you have dynamite, then bribe a cop they usually stands outside the bank. Then enter the bank and kill the first guard(shotgun is the best weapon on this task) then go to the vault kill the two other guards and place the dynamite on the vault.
Take cover, go and collect all the money and run outside take a car and drive to your safehouse fast. And that's how you make 4000$(minus 250$ for bribing the cop). But remember you can rob it several times.
That's all for this time hope you liked it and Good Luck in playing the Godfather.