you prolly have either a steering sector issue or steering linkage problem... or a worn steering shaft lower bushing.
Suggest getting the full model # and serial # off the ID plate/decal in the cavity under the tilting seat, do a google search for "MTD manual archives", enter required data, d/l free parts lists with diagrams and other relevant documents that will help you to understand involved components.
You will need to remove the mower deck to gain access to the chassis underside where the steering gear sector is located.. and you will need an assistant to help determine worn components causing problems that need replacement.