To start out there's only one main method to get money.
Firstly you'll need to mine for some diamonds, each diamond is worth $10 ingame!
Once you have some diamonds you can head to spawn (/warp spawn) and then just right click the slab with the floating diamond! Then just press the iron ingot and you'll see the diamond disappear from your inventory and $10 appear in your balance, press the iron ingot until you have all the money you need!
If you need the diamonds back just click the gold ingot and it'll take $10 from your balance and give your diamond back! You can't do this if you already spent your $10
Not everyone likes mining, I get it! Whilst there are no other official methods you can always try selling other items to players unofficially. You may have an item they want, such as an enchanted book, which you can sell to them by
/tpa name to that player and throwing them the item, they will then
/pay amount to you, or throw you diamonds you can then sell at spawn.