How to power the Arduno via Vin

this is my first project.
i understand that the Vin is 7-12V. however, all the rechargeable Polymer Lithium Ion Battery available is 3.7V.

How do i use a rechargable 3.7V Polymer Lithium Ion Battery - (400 or 1000 mah)?

I have to step up? How do I do that?

Also, i can connect the battery to a LiPo Charger Basic - Micro-USB, then step up to the board right?
And I will still be able to charge it?

Can you try two batteries in series? 2 X 3.7 volts = 7.4 volts.