It depends on how uniformal your faces should be. So far Blender cannot automatically retopologize meshes automatically in a way that the result would contain only evenly remeshed quads.
There are some addons for Blender which might be helpful with the task, "free" ones like this and also paid ones like Retopoflow.
Another free tool you might want to try is Instant Meshes.
If you don't need to have square faces, you can try the Remesh modifier. Depending on the topology of the source mesh, it also might need some work after applying.
My original mesh looks like this:
Go to the Properties Area, select the Modifier Panel, click "Add Modifier" and select "Remesh".
The Octree Depth controls the amount of detail and Scale defines the size of the grid. Depending on how much sharp details your mesh has you want to choose between "Smooth" and "Sharp". "Remove disconnected Pieces" might be handy if you have lots of "Ghost" pieces floating around.
Click "Apply" to finalize the remeshing process. You should then have something like this, which still has unevenly sized polygons but at least they are all quads and the amount decreased significantly.