There are various of ways on how you can make Gp in F2p on RuneScape. If you are a new player and in need of quick cash, i suggest you head towards the 'Stronghold of Security' and obtain at least 10k on the last level by completing all levels inside the stronghold.
Another way that you can make (some) Gp on RuneScape, but may not be fast depending on your mining level and how many supplies you have in order to make this item is to 'mine clay' and turn them into 'soft clay', then sell the in the Grand Exchange for market price.
If you have a high woodcutting level in F2p, then i suggest you try to cut some Yew logs. It may get boring from time to time, but try to do other ways to make coins.
For more information about the 'Stonghold of Security', click link to RuneScape wiki:
For more information about mining clay and soft clay, click link(s) to RuneScape wiki:
For more information about Yew logs, click link to RuneScape wiki: