Re: do i need to fully charge my samsung tab s2 before I start using it? Thx
No. You're fine. Try to keep it above 10 - 15% and fully charge rather than top up by 20% or so, though there will be times you will need to, and even going to zero and shutting down is not going to harm it occasionally (it will keep a little in the battery even then).
Best thing if you can is to regularly top it up to 95 - 100 % from around the 50% empty mark. This counts as half a cycle and there is less 'stress on the battery at that mark, for want of a better word.
Try the highly recommended GSam battery app for more detail.
Also the device allows a slower rate of charge when the device is low, or from 95 - 100 % so expect it to charge faster in the mid range where the battery also stays cooler, hence 50 - 100 is good.
If it's a used device, it's a different kettle of fish, but try try doing a full discharge from 100 to around 5 - 15% a few times to recalibrate it, then try to stick to 50 - 100%.
There are oodles of varying opinions, mine is an uneducated mish mash of what I read, but my mommy says I'm always right.
Try this guide by Rukbat as well.
http://forums.androidcentral.com/showthread.php?t=500054