I recently found out that my oil tank has a couple holes and has some water in it. I've been contemplating on how to go about in switching the heating system and how much will the conversion cost. Would someone be able to shed some light on this subject.

Thank you.

A lot depends on where you live and if you have insurance on your tank. Hope you do as leaking oil tanks can cost bundles to fix.
First off, you need to find out where the leak is. If it is the fill pipe which is the most common spot. Simpy fix the pipe and pump the water. If the tank has leaked and your considering changing fuel. Beware!!! Assuming you have insurance on the tank. Almost all tank insurance companies will have a clause that you remain with oil if they pay the cost of the cleanup and tank change. If they find out that you converted over to gas or another fuel source they will come after you for the cost of the cleanup and tank repair. If your considering changing what you will need to do is insist on having an obove ground tank installed to replace the inground tank you have. After about a year or so, this depends on your insurance, then change over to gas. It is then simply a matter of removal of the above ground tank.
When you decide to convert fuel you must take into consideration all the costs involved. such as chimney, tank removal, new fuel piping. With todays energy efficent systems both with oil and gas being very much the same. It may take you several years to pay off the cost of conversion. Also if your heating your home with water or steam, Stick with oil. It heats the water much faster and much cheeper then gas.
Hope this helps.