The bus system in Reykjavik is a little complex (by the way we do not have any trains or subway system) but you can figure out your way here. Usually what you do though is to find a bus stop and just ask the driver of the next bus for directions (or the local people for help). Note: You need to pay with cash in the bus and they do not give you change back so you better have the accurate fair (which is ISKR 350 per person / per ticket or about $ 3).
Taxis are easy to get in the center of Reykjavik and from any hotel / restaurant. If you are two or more and not going distances then Taxi is an easy and comfortable choice (specially in the evenings as the buses do not go after midnight). You can call Taxi at : 561 0000 and / or 588 5522.