Well first, in order to be able to help ya, we need to know what your skills are and what level they are.
Anyway, this might help ya:
This is really simple money-making after you reach 40 fishing and 40 cooking (in order to fish and cook lobsters).
If you're not 40 fishing/cooking, then you'll obviously have to train it up. From 1-20 fishing/cooking, you go net-fishing near Draynor or somewhere, fishing for shrimps and cooking em as well. After that, from 15-40, you'll need a lot of feathers. You can get feathers from killing chickens and you'll probably need anywhere from 1000-5000 feathers, though having around 3000 feathers would be a good number to start with.
Now that you've got feathers, you need a fly fishing rod. You can fly-fish trout at 20 fishing and salmon at 30 fishing. You can fly fish at places on the river near Barbarian Village and you may wish to cook these fish on the spot, or cook them on the range upstairs in the Edgevile General Store and then bank em (so you can later use em or sell em)
Once you've reached 40 fishing/cooking, head off to Karamja, and there will be a fishing platform near the Banana Plantation. There you can fish lobbies.
Though you can cook lobsters, you'll tend to burn a lot of em since 40 is the minimum cooking level for lobsters so it's better just to fish raw lobsters and bank em. Once you've got a good number of em (say 500-1000), sell em! I'm not too sure how much raw lobbies/cooked lobbies are these days, but you can try and find out in other websites or by asking someone.
Mining can be profitable once you've reached 30 mining. At 30 Mining, you can mine coal and coal sells for 100gp each. You can mine coal in many places, such as the Dwarven Mines (scorpians thrive here - be cautious if you're a low level player), Mining Guild (this requires 60 mining to enter)
and near the Barbarian Village.
As I said, coal sells for 100gp each, so do the maths. If you mined 100 coal, it can sell for 10,000GP. 1000 Coal --> sells for 100,000GP.
Bah, that took a while. That's all the memories I can recall when I was first started playing RS and making money so there, I hope I've helped ya.