Hi Michelle
To drive to the Blue Lagoon, you need to turn right on a bend about a mile short of the Hotel Mariposa, you will be on a gravel track (perfectly ok for a car), follow this road for as far as you can go, (about 2 miles), don't turn off on any of the tracks (BTW the first turning on the right takes you up to a great viewing point over Alhama), you will be driving next to a quick flowing canal, I think you go over a small bridge and keep going and you will come to viewing point for the lake, you can then follow the track down to the lake, you will have to walk the last 100mtrs.
Its stunning
Cheers Andy