Greek characters require a few manipulations (loading babel and using \textgreek{}), but for accents you literally just need to add two lines to your preamble:
Or you use XeLaTeX and you don't do those things. You just write Greek and say you want to use whatever normal font you like best for typesetting English and Greek at the same time. Because the "inputenc" package shouldn't be necessary in 2017. For more than obvious reasons, _everything_ is now utf8 encoded, including ascii files. Unless you live in a CJK country. Then you might have a reason to still use BIG5 or JIS... although that reason is slowly eroding in favour of UTF8 too.
