Metro from Las Ramblas to Parc Guell is really easy. Just take Line # (Green) from Placa Catalunya to Lesseps (direction: Canyelles)
If you're visiting the park by metro be prepared for at least a 20 min walk. The last 200 m walk is up a steep hill. If you have difficulties going up steep steps then a taxi or bus may be a better means of transport. You'll find bus information at the tourist information office at Placa de Catalunya.
Metro stop "Lesseps" (Green Line, L3) On leaving the metro follow the street signposts for the park
Open hours form 10:00 (closing time varies for different times of year but is normally around 19:00).
Entrance Fee - Free
Here is a map of the metro
As
GK mentions, the 10 trip ticket is the way to go. Easy to get around the city. A beautiful walking city.