To switch: Either click and pull down from 'ENG' to 'ΕΛ', or use the Windows + Space bar combination to toggle back and forth.
Many Greek letters correspond easily to English: 'α' is 'a', for example. But some do not. Here is the usual keyboard layout:
I have included the semi-colon for 'Q'. The semi-colon is used instead of '?' when typing a question in Greek.
Next time, step 3: The key combinations for breathings, accents, and so on.