Uhm, you basically sell the things that you have many of or you don't need.
You make automatic farms like Cactus farms, iron golem farms (these are the used farms because custom drops are disabled at the moment), and many more. You can watch a tutorials on youtube on how to make grinders / automatic ones.
You can go to profit shops too by warping to people's warps. you usually see people advertising their shops in chat. Also, you can do /is warps to see all the warps of the players, you can see what their warps are - either it's a profit shops, EXP grinders, a scenery warp (for just viewing their island and such), and so on.
Just like in Prison, they have prestige levels and stuff while on Skyblock, its island levels. You level up by expanding your island. then do /is level for it to calculate on how big your island have progressed. To know what block is better to be place and how much it will give you, do /is value while holding the block.