To prove that your Turing Machine is undecidable, you must construct a decider for it and show that if your decider works (this will lead to a contradiction, so a proof by contradiction is used), then a known undecidable problem could also be decided as a result. Three known undecidable TMs related to your problem are the "Turing Machine that accepts a certain string w", the "Turing machine that accepts no strings (empty set)", and the "Turing machine whose language is a regular language (i.e. has an equivalent finite automaton)".
For example, for the "Turing machine that halts on input w" problem, we can show that it is undecidable because if we had a decider for it, then the "Turing machine that accepts a certain string w" would be decidable as well; a contradiction. This is so because if we had some way to detect whether a Turing Machine halts on input, then the Turing machine could simply reject when it loops, or accept if the input is the wanted string w; thus becoming a decider as opposed to a recognizer. Since this decider would ultimately help us construct a decider for a known undecidable problem, it cannot exist; thereby proving that our original problem is undecidable.
I know it's very confusing. Took me a while to understand it.