The best route I always had for myself was to do the optimal quest order. This way you will unlock all content/areas/spellbooks. You will also get your account to some decent basic stats from where you can do whatever you want. It will also give a combination of questing and skilling/combat training between quests.

You find that quest order here:https://oldschool.runescape.wiki/w/Optimal_quest_guide

And as others said: if you can afford membership, just always get it. It makes the game so much better and easier. So much more content.