As stated before, often it is asymptomatic. The primary way of contracting Chlamydia is unprotected Vaginal, Anal and Oral sex. The most obvious way is to use protection such as Condoms. Limiting the number of sexual partners greatly will reduce the risk of contracting Chlamydia. If you do have Chlamydia, it is best to tell your partner so that others don't get it, or you yourself don't get it again after you've been treated.
Treatment includes various antibiotics.
Penicillin does NOT work. Rather, erythromycin, azithromycin and ofloxocin are used widely.