Confused about long and short

I am new to forex. Still cannot figure how to go short. My problem is this.

Lets say I have USD in my account. I want to go long on GBPUSD which means I buy GBP and wait for the price to go up and then I close with some profit. So my the amoutn of USD in my account increases.

The next day, I want to go short because GBP is going down and I want to sell GBP with the intention of buying them back at a lower price. This is going short? Right? How can I do this when there is only USD in my account.

Thanks.

Your not selling the money in your account, your selling the banks GBP and buying dollars. The money in your account is only to cover the margin necessary to hold the position.

Open a demo account and just sell something. It will make perfect sense...

Thanks,

I now I understand.