If you want to make money, here is what I did, and some others when I helped them.
Firstly start off a wheat, or sugarcane farm. The give good money for starters and if you want to, you can carry this on in the future. Cocoa beans are also great because they provide amazing profit and the literally regrow instantly. I would avoid cacti because it has been nerfed, and the selling price is 0.5 cents. 300 cacti = 1 wheat, so that's why you should avoid it. Carrot and potato farms are also quite good as they give much more when harvested, but personally I didn't do it, I stuck with sugarcane and wheat.
After that, you may get rather bored of farming so you can get spawners. You can get cow spawners in the beginning because they sell for the same price near creepers, and are rather quite cheap. After cows, then you could progress to creepers and they are great, then iron golems and if you are rich enough, then evokers. Iron golems are very good as they give good money and aren't as expensive as evokers. Possibly at the end of skyblock then you could replace iron golems with evokers.
You can also take part in coinflips, where you can either win double the money you put in or sadly, lose it all. Island quests are also a very good way of making a quick buck and also you could farm heads for players. At the time I am writing this, a stack of heads sell for about 750-800k. That's not bad.
Hope this helped :D
Eureka14, a fellow Skyblock player.