Quote from EquityGuy4321:
At any given point in the life of the contract one bond in the deliverable basket will be the cheapest-to-deliver (if you had to deliver a bond to someone, and you had a choice, you would deliver the one that would cost you least, right?). CTD is influenced by a number of things, such as current financing costs and the composition of the deliverable basket, but unless you want to take or make delivery (which almost no one ever does) the only thing you need to know is that the futures contract will trade like one bond... namely the CTD, and most of the effect will be in the general up/down movement of rates, so it is a second-order detail which bond is the CTD. The one thing that CTD does tell you is the DV01 of the bonds and, hence, the futures contract. If you are concerned about drift in the hedge ratio then you might want to keep track of the shifts in CTD (if there are any).
If you want to short a cash bond you have to find someone who will lend you the bond in order for you to sell it. This is a repo. Works much the same way as shorting a stock. So, you have to have an arrangement with a prime broker who will do this for you. Merlin Securities might be worth a shot since they specialize in smaller accounts, but you still have to have a good chunk in order for it to be worth their while.
I suggest you read up on LTCM and such since they famously blew up on these types of "convergence trades".