1. Settings/General/Storage. Tap Cached Data and the data cached by apps will be cleared. Then restart in recovery and select the Wipe cache item (and press the power button). That will wipe the system's cache.
2. There's a timer for the lock screen in Settings/Display/Lock Screen/Secured lock time. If you have the screen locking quickly, that will do it. For safety, it should lock about 30 seconds after you last use it. But for convenience, it shouldn't lock at all when you're home.
Tasker makes it possible to switch times depending on where you are. (It has nothing to do with storage. The message is talking about
internal storage. Put all your media files on the card to free up some internal storage. (The phone has only 4GB of internal storage, which would have made it a small phone in 2013, and there's no way to increase that.)