If you’re not into coding and fiddling with template layouts, I strongly recommend Z-Blocks from Aheadworks. It kind of turns Magento into a Joomla-like module system and it’s great!
So to add blocks you might need to do two things depending on your theme. One is to add the block to your page or catalog xml file so it get’s “loaded”. Then, if your template doesn’t have this block in the php, you’ll need to add a line to call the block you loaded. This sounds complicated, but it’s very powerful and extremely flexible.