The first thing to do is to keep it wet, either in salt or fresh water. Fresh is better, but keep it wet.
Next, there is a lot of growth and concreation on iot. This forms a shell around the iron and needs to come off. Most times a blow from a hammer or mallet will break it.
Once all the concreation is off, keep it wet again. It looks like it came from salt water, if so, keep it wet for anywhere from 6 months to 2+ years in fresh water to leach out the chloride ions. If you want to do electrolisys, we can tell you how, but it needs to be cleaned up first.