You need what is generally referred to as a "light distro." There are many to choose from, and I think it's best that you spend some time investigating what feels
right to you. Picking a distro is a philosophical and personal choice. All you can get here is people advertising their favorites. None is distinctively superior to the others in any technical sense.
Here is a starter list.
When one looks interesting, check out the home page.
Here's a Distrowatch search
for much the same thing.
Read through those pages. Look for interesting links as well as the primary reviews.
And search these forums for "light distro" or "old computer."
This question is asked very often, and all the answers are already posted. You really should have just read one of the other 10,000 or so threads with the same title.