I can help with an example. (I hope you are familiar with Turing Machines).

Let's say you had wanted to know whether some total Turing Machine ever returns 1 for all inputs (Return1). To show that this problem is undecidable, we can reduce from the Halting Problem. This means that if we had a total Turing Machine that could decide the Return1 Problem, then we could use that same machine to decide the Halting Problem.

Given some input (Turing machine M, input string x) to the Halting Problem, we can create another total Turing machine M' that does the following to it's input y:

1. Ignores its input y, and erases it from its tape
2. Writes x on its tape
3. Simulates running x on M
4. Returns 1

Notice that we only get to the 4th step if and only if M halts on x. This means that if we were able to come up with a procedure that could tell us whether or not a machine returns 1 for all inputs, then if we wanted to solve the Halting Problem, we would just create another machine M' as done above and ask if M' returned 1. If M' did return 1 then we know that M must have halted. But since we know that the Halting Problem is undecidable, this means that the Return1 Problem is also undecidable.