Someplace where it wont fall (i.e close to the ground) in a basement is fine.
Only if you don't want it to fire. Actually it is best to store the gun without a C02 powerlett in it.
If you are not trained on filling Co2 tanks (unlike HPA tanks) you should not attempt this, on risk of breaking the burst disks, or freezing your hand. An HPA tank however, you can simply hook up and fill.
Dicks, any paintball shop, and Gander Mt.
Take it to a shop! if you want to do it yourself the normal way is to put the bottle on scales and fill it from a larger bottle to a fill weight. To be honest though CO2 is well out of date. A smart move would be to move onto compressed air.
No paintball gun will work without a CO2 tank or other propellant tank. without Co2 there is nothing to push the ball out of the barrel.
If it has threads on the top, you refill it like any other tank by taking it to a sporting goods store or paintball field. If it does not, then it is a disposable, and you cannot refill it.
A CO2 tank does not necessarily have to be upright. It depends if you want liquid CO2 or gaseous CO2. When I purchased a CO2 tank for a specific application, namely, CO2 snow cleaning, I had the option of getting a tank that either had a dipstick, that picked up liquid CO2 from the bottom of the tank, or one that had no dipstick so that it collected gas from the top of the tank. I selected a tank without the liquid pickup. So, my upright tank output gas. However, if I wanted liquid CO2, I would put my tank slightly upside down, and get liquid CO2. It is probably not recommended to run a commercial gas tank in a non-upright position. So, depending on what type of tank you have, and what type of output you want, the answer to your question literally is: depends. For farm and garden use, you are probably seeking gaseous output, and probably have a tank with no dipstick; based on these assumptions, yes, you will need to keep your tank upright.
No co2 tanks are sold full.
"H" is not a size for CO2 tanks
There is no such thing as a "CA" tank.
Typically it is around 7-10 dollars. Depending on the store.
You cannot unless you have a gauge installed, or know the exact weight of the tank.