Ummm, how to say this politely - your LBS is full of it. You have traditional forward-facing horizontal dropouts (slightly angled to keep the brake pads positioned correctly no matter how far forward or back you pull your rear wheel). In other words, you have exactly the right dropouts for a single-speed or fixed-gear conversion. You do NOT need a chain tensioner. Frankly, if you took it to your LBS for them to look at and they looked at it and told you this, you need another LBS. Period.

With that out of the way, randyjawa's guide gives you the information you need - if you desire it, you can trawl Sheldon Brown's writings on fixed-gear conversions would augment that.