Purchase a home Greek language training course. Popular varieties include Rosetta Stone and Topics Instant Immersion software.
Study the material daily. Practice vocabulary and sentence structure in both the verbal and written form.
Sign up for a Greek language course at your local university or community college. Depending on the area of the country you live in these may be common or harder to come by.
Converse with local Greek immigrants or those of Greek heritage. Local ethnic communities often have gatherings or community centers where this would be possible.
Take a trip to Greece. Being immersed in the Greek culture and speaking their language is the quickest and only sure way of language acquisition.