We did this day trip a couple of years ago by taking the bus from Benalmadena to Malaga bis station, then changing there to the Nerja bus.

http://www.nerjatoday.com/bus.htm

Have a look at this link which gives bus timetables for Malaga-Nerja-Malaga
Also gives you length of that leg of your journey.

http://www.gomarbella.com/costadelsolbuses/benalmadena-costabus.pdf

scroll down to page 3 of this link for buses btween Benalmadena and Marbella.
Dont forget when you get to Nerja, to take the local bus up to Frigiliana, one of the pueblos blancos (white villages). Its beautiful.
Good luck

You might want to book an organised tour to Nerja, from the likes of Arman Travel (see link), near the Bali Hotel. Current price is 13 euros.
http://benalmadena.homestead.com/cdsol.html

Thanks Jade , I wil have a look at bus times.

Thanks Tony I have a leaflet somwhere that didn`t have price on it. I will have alook a website.

Chris if you do book as an excursion, include a visit to the the caves, well worth the extra few €s