I just bought a couple of the deal extreme japod ones. At that price you can afford a couple of dodgy ones I reckon.

I would recommend getting a charger. With three batteries I can have one in the phone, one in the charger and one in my pocket. I am using the charger I got for my n810 batteries. The n900 batteries are smaller but the contacts are the same.

First time - charge it until it is full. Li-Ion doesn't suffer from memory effects, but does suffer from performace loss through shallow charging. So charge it to full, from (near) empty as often as you can.

I am assuming that you can use while charging, I expect that the power is sourced from the charger rather than the battery when plugged in. I could be wrong though.