Slayer is your best friend, always go to the right slayer master for your level as well. https://oldschool.runescape.wiki/w/Slayer_training
On F2P kill cows and other shit until 30s and then go onto the barb village until 50 probs
p2p do slayer
You could also refer to the guides section of the runescape sub forum on here and see if theres anything that helps you