First battery charge on new tablet? I got yesterday a Galaxy Tab 3 and I want to be sure what's the best way for the first battery charge. On the shop, they told just turn it on, wait for the battery to finish and then charge it to 100% (3 times!).

When I got home I turned it on and the battery was at 50%. But then I saw on the little book in the box that said: when you buy a new tablet, or the battery has not been used for a long time, charge the battery.

So, what do I do now?

The tablet is in sleep mode with 20% battery.

Should I turn it off and charge it? Or just charge it as it is (on)?
there is no significant difference between charging it while it's turned off or not.
Modern Li-Ion and Li-Po batteries which are built into tablets and phones nowadays can handle any kind of charging very well. The only thing that should be avoided is a complete discharge as it degrades the battery MUCH faster (aka it loses capacity). With discharging I mean draining it to 0% and leaving it for some days/weeks alone.

TL;DR: Just use your device as you want and avoid being on 0% battery for a long time.