Don't you need to start again in order to play old school?
If you wanted to play Old School, you would need to start over with fresh stats, bank, and everything. But if you do, there are lots of awesome people like I Love You who can help you out with any tips or questions you need.
Assuming for the moment that you want to continue on in RS3 though, I would probably recommend doing Slayer. This will help you finish off your combat stats (sans prayer) and make you a decent chunk of change as you train it. Then you can use that money to progressively upgrade gear followed by more expensive (but useful) skills so you can get into higher level PvM/bossing and make even more! If you don't want to do Slayer or want other things to do to break up the monotony, you might choose to start levelling up your Runecrafting or Divination skills. Fishing sharks is generally a really poor use of time as you can do so much more and just buy the food instead. If you want to get going I can give you a few sharks so you can get started or you can get a cheaper food like swordfish or monkfish.
In terms of gear progression probably rune/dragon pieces as you can afford them is best for now, upgrading to bandos as you have funds available. There is also warpriest available as occasional drops from God Wars Dungeon mobs, but I'm not sure it would be productive to farm them with lower levelled gear.
If you would like to message me/need more info feel free to reply here or message me ingame (Arceuus). If I don't reply I am a little more preoccupied than usual, so hopefully no offense taken!