Harper Lee creates tension by using characterization and diction to begin. Scout our narrator notes about Jem that "He sounded fishy". The boys want to go get a glimpse of Boo and they use their male superiority to make Scout feel like she doesn't have to do it if she is scared. They also try to keep it a secret for awhile. Anytime an author can keep a secret from the audience makes us wonder what it is too.
Lee uses sounds (sensory detail) in the porch swings creaking and the old people on the street groaning. Then she uses sights at the Radleys' house: a loose shutter, and a "hat-rack mirror [that] caught the moon and shone eerily".
Lee also uses time. For Scout, they moved so slow that every move felt like it took her an entire minute. When things take longer, this builds suspense, and therefore tension.