If you plan to rent a car you should get a 4x4 vehicle, not only for the traction but for the clearance as well - you don't want to return the car minus the front end as a result of scraping along the "roads" in Monteverde. First off, you need to understand that Costa Rica's only complete north-south route is the Pan American Highway, also called route number one. This road enters Costa Rica from Nicaragua and runs south on the pacific slope of the country, passes though San Jose, and continues on to Panama. If your heading from San Jose to any place north of Puntarenas (Guanacaste and beaches on Nicoya for example) you will follow the Pan American.
This is an exit on the Pan American highway at the Rancho Grande Gas Station which takes you 3km to Sardinal and offers pavement all the way to Guacimal. This is the route to take if you're coming from San Jose. Driving from San Jose to Monteverde will take about 3.5 hours in total. From San Jose to the Rancho Grande turn off it is about two hours and from there it takes about 1 hour to get up the mountain to Monteverde. Traffic will influence the travel times especially the parts near San Jose. Certain times of the year or days of the week there is an increase in travel and this crates a clog in this artery of Costa Rican transit.
If you're traveling between locations in northern Costa Rica and Monteverde you may chose to come via Las Juntas de Abangares. Las Juntas is located just off the Pan American highway at a turnoff called "La Irma" after the gas station located at this junction. The road from La Irma to Las Juntas is paved but from Las Juntas to Monteverde the road is unpaved and allow around 1.5 hours.
this is the road that you will take if you're coming from Arenal. It is unpaved and joins the road coming in from Las Juntas as you get nearer to Monteverde.
Lagartos is an exit on the Pan Americana located between Las Juntas and Sardinal. This is the route the public bus takes and is not paved. If your driving there is really no reason to take this road and you might as well travel further south and take the Rancho Grande gas station turn off to Sardinal. Drivers should plan to travel during daylight hours as there are dark unpaved winding roads that sport sheer cliff edges.