I recommend entering the park at the entrance at the top (sorry don't know the name of the entrance), from here you can walk down hill/down stairs to the main entrance. If you enter from the main entrance you'll be walking uphill/up stairs to see the park.

Do you want to visit Sargrada Familia from inside? If so, check their opening hours and book online to avoid the long lines to enter.

Unfortunately, the morning is the best time to visit Park Guell, Sargrada Familia and the Boqueria Market (don't expect all the market stalls to be open in the evening).