Your problem is probably with the chain. A cheapo chain is maybe $15. Many folks suggest changing the freewheel/cassette at the same time, but then it gets expensive.Just change the chain- you will need a $10 tool if you DIY- and call it a day.
Plan B don't use that smallest cog-save the $$.

In respect to posting for answers that might be found somewhere else; the OP is new and trying to make a connection to the forum members. He certainly isn't looking for some sort of English lesson.
Some members need to learn some manners.