I'm not going to give away too many trade secrets but I'll give you a workflow that will help make things sound better.
First thing you have to do, nooj, is clean up the production sound. You need to eq out noise, get rid of clicks and pops on dialogue, kill on-set fx noises (squibs, explosions, lights falling on gaffers etc) and get the sound to a listenable quality. That is to say, room tone and dialogue and not much else.
Then you add a layer of BGs, sometimes called ambis in the UK and Australia. So, things like room tone if you're inside, the noise of an office, phones ringing etc. Or if you're outside you'll need birds, car passes, crickets (or cicadas if you so prefer). These will get a final mix from the Re-recording mixer but the BGs editor will do a preliminary leveling, add some reverb and some basic eq-ing. BGs should take up between six and twelve channels in your mix. (3-6 stereo, 3-6 mono)
FX (or effects) are done simultaneously by a different editor (usually). These are specific, as seen by the audience, sounds. It's the old mantra "See a cow, hear a cow!" Punches, explosions, car-bys, Transformers transforming, Crispin Glover's eyes sizzle are all fx.
The key to really good fx is the mix. Again, the FX editor usually does a preliminary mix blending the sounds together using a really good equalizer, some sort of convolution reverb (to make the fx sound like they exist in the same space as the dialogue) and lots of leveling plus a scattering of fades. The mixer will finesse all these elements using a large series of tools and plug-ins used in such a way as to confuse mortal men and scare directors.
Then, if you're feeling like you want to you add some foley (human body sounds) and record some walla (basically background voices) to sweeten everything.
Here's the big key though, for a good sounding film with great fx, it's the little things. A huge explosion is great but it's the falling debris, the sizzle of burnt flesh, and the groaning of metal that will sell that explosion as dangerous. A creaky floor signals tension but the squeak of the leather of the shoe plus that hint of reverb on a very empty room makes it significantly better. Those little things are what separates great films from fucking brilliant.