Start with high volume items, ones that have buy limits of 7-30k each, and are traded very regularly on the ge. Use GE Tracker and the osrs GE website to find the most traded items. This way, if a flip goes south, you usually won't lose much while still having the potential to make bank.
Aim for 35-50k+ total profit for each ge buying/selling slot. E.g., if an item has a buy limit of 13k, and you get a margin of 3gp, that's a decent profit since the flips usually go pretty fast with high volume items. It adds up and you'll get the hang of things while exploring more items until you find some that work repeatedly for you
At first, shoot for high volume items that are less than 1500gp each, as these tend to fluctuate the least and pose the least risk.
Good luck, I think it's the best way to make money in the game, and is where 85%+ of my bank has come from.