There are various choices if you are interested in learning Greek as a foreign language.
Multiple sites offer online courses for beginners, intermediate and advanced users, with highly variable costs or even for free.
The introduction of new technologies, such as the smartphones, has also provided new platforms, and among others, apps that aim at teaching Greek and other languages have been developed.
There is always the choice of private tutoring, of course. Depending on your needs, you may choose one of the aforementioned choices or many others. Just bear in mind that Greek is a difficult language to master and considerable effort is needed to learn!