Using decision version of TSP to solve optimization version
I'm not sure if we should be answering such questions, since it is at the level of a homework question, and not a research level question. So instead I'll just give a hint to the OP so the OP can figure out the answer with some effort. (Other posters may choose to give out the explicit solution if they wish, of course.)

The concept you refer to, i.e., reducing the search problem (or optimization problem) to the decision problem, is called self-reducibility. It is known that all NP-complete problems, and some others like Graph Isomorphism have this property. See this powerpoint presentation on self-reducibility. It explains in detail how SAT, GI and NP-complete problems in general can be shown to be self-reducible.

If you read those notes and understand those examples, you should be able to do the same for TSP using similar ideas.  Hope that helps.