When you get it, fully charge it, and leave it charging for a few hours more than it needs to be fully charged.
When storing it (i.e., when you're not using it), keep it mostly charged, but not powered if you don't want to.
When you start to use it, if it needs power and it's convenient then connect it to the mains as you're using it to keep it fully charged for next time.
Every few months fully discharge it (by just leaving it on for a while), then once it's stopped working for a few hours due to an empty battery, put it on charge and leave it on charge for a few hours after it's become fully charged (like you did when you first got it).
This will help to keep the battery as good as it can be.