Hi Aina, thanks for writing in. Have a look at the link to the Avanza bus company timetable at the top of the Gomarbella Costa del Sol buses page. Check the Ida button (One way) then select Marbella as your Origen and Cadiz as your Destino, then your date of travel and you'll see the cost and the different services available.
This route is operated by the Comes bus company based in Cadiz and the bus you will catch actually starts from Malaga bus station, stopping at the larger bus stations on its way down to Cadiz.
Why I am explaining this is that if you are travelling at a peak time, like Friday afternoon, it would be a good idea to book it online in advance or go up to the Marbella bus station and book your seat, you can do this up to 7 days ahead normally. Or at least do it a few hours before because it increases the chances of getting on.
Having said this, a couple of years ago in summer we booked tickets for friends arriving from Africa and travelling onwards from Marbella to Cadiz. We booked the tickets two days before the bus left. They actually got seats 1 and 2 but when the bus pulled in from Malaga, it was full and one friend had to sit on the jump seat and the other sat on the steps all the way to Cadiz. This was not quite what they were hoping for.
The bus to Cadiz stops in Algeciras for a coffee and loo stop before pulling back on to the coastal road for the final leg.
Just outside Tarifa town as the bus heads for Cadiz the sea will be on your left and above the roofs of the beach bars and hotels you will see the coloured sails of kite surfers high above the sea. This scene below is looking from the road over the 100 % fun beach bar and kite surfing school.