For most bus stations in San Jose, you can simply ask a taxi driver to go to the "estation de bus a La Fortuna/Arenal" or whatever destination you want, using LaFortuna/Arenal for an example. The taxista will take you to it no problem although having the address written out is always a good backup. Keep in mind, the company that runs to La Fortuna is NOT TicaBus. If you ask to go to the TicaBus station, you won't be at the right place.
Buying a ticket is no problem. There will be a standard 'caja' where people are in line to buy a ticket from someone behind some glass and walled in. Just say 'uno a La Fortuna' and they will tell you the cost. Give 'em some money, you get your ticket and change. If the bus station has more than one departure city, there will be large signs indicating an area for your destination (directo doesn't have stops...sorta) with a bunch of chairs which is the line to board the next bus. Sit down in line and when everyone else gets up to board, follow along. Sometimes the bus driver will take your ticket (plastic card) as you enter the bus and sometimes after boarding, he comes by and collects them. Keep an eye on your bags...
On arrival you will be on the south side of La Fortuna with some more budget hotels nearby but most nicer ones a taxi ride away. There is a grocery store right next to the bus station if you want to stock up on stuff for your hotel room and taxis are easily available and will take you to any hotel you name...again, written down is best. The bus ride will be around 3-4 hours depending on the road and traffic. It won't be fast but there will be ample opportunity for photos out the windows of the bus...unless it is raining and the windows all fog up OR the bus is inside cloud the majority of the trip which is a good possibility this time of year.
Keep in mind, these directions are more for longer journeys outside San Jose and the suburbs. Most city buses have no station, only stops and you pay the driver as you board. The final destination where they basically turn around on a loop is listed in the windows along with significant stops it may make.
Hope that helps and great to hear you are taking time to get and and see the country...something I always suggest! You will enjoy the Arenal area with a ton of things to do. If you haven't, check out
http://arenal.net for a bunch of great suggestions on things to do, see and places to stay. And as a caveat, I have nothing to do with that site just find it a useful reference...