Don’t use an old guitar that’s been sitting in the attic for 10 or 20 years.
Chances are it will be “clunky” amd hard to play – too big and cumbersome, with corroded hard-to-press-down strings, and lousy “action”.
“Action” is the term for how easy it is to press the strings down. If they are too high up off the fretboard, the guitar will be a lot more difficult to play than it should be.
Using an old hard-to-play guitar is the best way to get discouraged and quit before you give it a realistic shot.