The first thing you need to do is prep your wood with a good sanding starting with 100 grit to remove any exsisting finish.
There are many different tools to use to distress the surface of the wood, some you can buy some you have to make but they are very easy to create.
This is a good combo by Varathane that costs around $25 for the set.
A wire brush is a good tool to rough up the surface of the wood grain with out gouging it or chipping it.
A small length of chain is good for beating the hell our of a perfectly good peice of wood or a pillow case full of differenct size nuts and bolts.
Once you have the desired disstressed look try staining it by applying a thick coat to the surface of the wood making sure all the disstressed marks are saturated and using a rag to wipe off the exsses from the surface. The dents, scratches and chips will pick up more stain thean the surface giving it a more worn look