I know some Koine Greek and am just starting on Hebrew, both of which I wanted to learn to help me understand the Bible better (as a Christian). For Greek, I bought myself a textbook to get started and did a lot of vocabulary on Memrise. To keep up my interest, I started reading "real texts" as quickly as possible, using a bilingual edition of the New Testament. It's worked fairly well, I can read and understand ~80% of the NT, though my grasp of the grammar is still thin.

I'm using the same approach for Hebrew now, hope it works again. Latin is also on my learning list because of its importance in studying history, but I haven't seriously gotten to it yet.