For a retail trader you need to use either Exchange traded...

For a retail trader you need to use either Exchange traded options, Mini warrants or CFD's.
In some cases retailers on HC have said they have been able to actual short sell a stock under a lending agreement but I don't believe this is common or easily accessed.

Each of the instruments I've outlined needs detailed study and understanding of their characteristics as you have a leveraged position which is much higher risk. I will never touch cfd's, where you can lose more than the capital put up and suggest you don't unless very experienced.

If you are new to shorting, the safest way is probably through Mini warrants. The main issuer is Citifirst and you can look up their website for details on the Mini Warrant product. Study it carefully. You can buy and sell these through any broker just like a share.

I trade with Options and occassionally Mini Warrants, as options have limited coverage of shares.