I want to use more of my RAM usage when I render at first it said I was using 40% but now it says Im only using 15%
There's not much you can do about it. AE will use as much memory as it needs to.
Ignore the message in the After Effects UI about how much RAM is being used. This is just telling you how much the foreground process is using at that moment, ignoring background processes, caches, and so on.