pedals, cleats and shoes??

Basically every clipless off-road shoe uses the same cleat mounting system. This uses two screws to hold the cleat to the shoe sole which is recessed into a "pocket". This means that the cleat does not protrude beyond the sole so that you can walk in relative comfort. Look for shoes that are "SPD (Shimano Pedalling Dynamics) Compatible".

There are three main manufacturers of clipless pedals. These are Shimano, Crank Brothers and Time. Several other manufacturers (such as Wellgo) make clipless pedals too which are about 99% compatible with Shimano. Unfortunately, Shimano, Crank Brothers and Time all use different cleats so once you've chosen your system you have to stick with it unless you are prepared to start changing cleats as well as pedals. So you can't use Shimano cleats with Time pedals or Time cleats with Crank Brothers pedals.

Shimano's SPD system is the oldest as far as MTBs go and it says something that other manufacturers copy it or use the cleat interface to mount their own. Like any system, each manufacturers has it's pros and cons.

Personnally I'd go with Shimano every time. I've been using them since they were introduced, I've never had any problems with them and I see no reason to change.

As far as shoes go, if you want to use flatties as well as clipless pedals go for a skate-styled shoe like 661 Duallies or Shimano DX. Trail or race shoes aren't really suitable for use with flatties.

Finally, here's a tip if you want to go with Shimano SPDs. They make two types of cleat called Multi Release and Single Release. Start off with the multi-release version as they will disengage from the pedal if you give them a good yank in virtually any direction. The single release version will only disengage by twisting. So the multis are best for learning on.