Find the source of the leak

If in the basement, usually the leak is from bathroom or kitchen. Easy fix.

If the roof leaks, then find out when was the last time the roof was replaced. If it is too old, then replace the whole roof could cost you a fortune. But, if the price is low enough, i.e., the difference is much more than the roof and other needed expenses, plus your time and energy, then you can still consider it.