Repair any conditions that allow water to puddle or stand at the base of your porch column. This may require adding flashing to parts of your house to deflect rainwater. It may also involve redirecting a sprinkler or sprinkler head that is constantly watering your porch column, or it may involve building up the wood slightly around the base of the column --- perhaps with wood putty --- to direct water away from the base of the column. The same is true for the top of the column --- do whatever is necessary to redirect water away from the end grain of the porch column.
Drill holes from the underside of the porch up into the center of the porch column if this is a possibility. Drill at least four 1/4-inch holes if possible to allow air to circulate into the wood of the column and to allow water an easy way to drain away. The idea is to keep the end of your porch column as dry as possible.
Poke the sides of the bottom and top of the porch column with a screwdriver. If the wood is rotten already and breaks away, then clean out as much of the rotten wood as possible, using your screwdriver. If the damage is already extensive --- say, 40 percent or more of the wood is already rotten --- then the top or bottom of the column may need to be replaced with new wood, or the entire column may need to be replaced.
Fill any holes in the base or top of your column with wood putty if the damage is not too extensive. A water-based wood putty may be your best choice, as these types of putty dry rock-hard and are resistant to water. Use a 2-inch putty knife to force the putty into the wood, and smooth the outside of the putty so it is level with the surface of your column.
Sand the putty smooth once it is dry, using 100-grit sandpaper.