Take the 41 from Fresno up to the South Gate of Yosemite NP. Follow the road into the Valley and park at Camp Curry. Follow the sign for Mirror Lake, easy walk along a paved road for about 1 mile.
Not a large place, at least in the summer, but the water is still and you get a great reflection of the background in it. I don't know how it looks in any other season, since we were there in mid-July.