The best way to get OSRS Gold is to earn it through the game. There are many great strategies for F2P and P2P money-making on YouTube as well as in the Runescape Wiki.
Personally, I recently crossed 10 million. One of my most favorite strategies include:
Selling and cutting yew logs 60 woodcutting (though this method isn't as effective as it was in the past).
Mining iron Very low-level requirement, and very fast, too. It's also the best method to get mining training in F2P.
Smithing/Superheating runite bars -- This requires very high stats, but is the best method in F2P
High level alchemy Visit the wiki page on the High Level alchemy market watch to locate the most valuable items.
Fletching arrows with no head It's quick and simple and with without any stat requirements. Simply purchase arrowshafts and Buy RuneScape Gold feathers then fletch them and sell them back to make an incredible profit.
These methods are all F2P. There are many more on the Wiki as well as many more are available to you when you're an active member. Enjoy earning money in a safe and secure manner!