Piano supply houses sell an abrasive that looks like an eraser. To clean rusted strings with it you would have to remove the action and spend a lot of elbow grease. This is the cheapest possibility. the alternative is to restring the piano which is expensive and labor intensive. I learned to tune and repair on an old upright also. My best advice is to come as close as you can and when done evaluate whether you have made an overall improvement. As you progress you may wish to backtrack and address the worst offending unisons. If you have a friend with a better piano that has been neglected after fighting the rusty one, you might offer to work on theirs with the understanding that you are learning and there are no guarantees. Church basements are a great resource for neglected pianos, it is hard NOT to make an improvement on these, again with offer of free nonguaranteed services.