There are many profitable money making methods, and it's rather easy to make cash on Runescape if you know what your doing. First of all, always do farming runs because you can earn a lot of money over time, and it is a fun skill (IMO). That will earn you some cash with some of the herbs you can plant especially at higher levels. It can cost a bit to raise it fast however.
spending a lot of time woodcutting is always too boring. Magic trees aren't bad to cut then make into bows for experience. I used to host flower games all the time in a trusted cc, and I made so much cash I don't really need to worry about it anymore. however that is against the rules now. Hunter is a good one too, plus a fun skill. I also enjoy grinding monsters, with the wilderness update all items drop on death in wilds, so if you want to take a risk go to green dragons in wild, not only do you get cash but combat exp, and no bots now. Plus you can PK people out there, but unless you are confident facing other 200 combats at the same time, with PK equipment, I wouldn't risk it.
Flipping items is what I would do now in the GE, but it takes time to learn how to do that properly. Watch some youtube vids on it. Current ones, some are outdated however the methods taught should still work properly. There are of course many other methods to make money, but those are just some of my favorites. Just train skills, make goals for skill levels, and just have fun on the game, cash will come.