You'll find the answer to that and much more here:
THE GREEK ALPHABET
see each letter
learn how it's pronounced
HOW TO GET THE GREEK KEYBOARD These links will not only show you
how to get the Greek keyboard but also
how to find the Greek letters on it,
how to add accents etc.
https://www.duolingo.com/comment/22040507 It will also help you learn the alphabet and where to find other important information.
THE GREEK FORUM WITH MORE LINKS
ι and η sound the same to me. Is there a way to know when to use one over the other?
Just memorisation, I'm afraid.
Much like English ee versus ea, the different spellings arose from when the sounds used to be pronounced differently, but now that sound changes have merged them, English school children have to learn that clean is spelled with ea and green with ee even though they are perfect rhymes for almost all native speakers -- and similarly, Greek school children have to learn that "afti" is spelled αυτί when it means "ear" but αυτή when it means "she" and αυτοί when it means "they".
If you speak Ancient Greek (where ι η υ ει οι were all pronounced differently), then that can help, but otherwise, it's mostly just memorisation.