Come out of the baggage reclaim area in the T3 terminal and walk straight across pass the lifts and ramps and the Malaga Airport bus stop is directly in front of you.
Here is a link to information on where you can buy tickets for the bus to Marbella.
Just beyond the bus stop is the Malaga airport Train Station
The Linea A Express bus from the airport stops at the Maria Zambrano train station and then the next stop is to Malaga Bus Station. It also travels through the centre of Malaga city.
This service runs every 30 minutes into Malaga from 6.30 am to 11.30pm.
About 5 minutes before the due time, the Malaga Airport to Marbella Bus Station bus will arrive and stop here. This services runs every day of the year but check through on the bus schedule page for the latest update.
If you are travelling to or from Malaga airport might find it useful to check your flight's arrival or departure times from Malaga airport also to see how they tie in with the airport bus schedule.
Hope this helps. Have a great trip Best wishes Mike