There is a good chance the single speed conversion kit will not work on the huffy. A huffy is likely going to use a freewheel and those kits are designed around cassette hubs. You can still use the tensioner on the multi speed freewheel but the tensioner may end up rubbing the cogs.

I will second the idea of trying to find a quality older mountain bike (like a 20ish year old Rockhopper) and starting with that. You should be able to find one very cheap on craigslist.