Salve! Well, you've come to the right place. Latin is my particular passion, and Greek my hobby. If you're looking for a place to start, I stronging recommend LINGVA LATINA by Hans Ørberg; I have used this book with my students for five months now, and the results have been very positive! I also learned Latin myself by means of LINGVA LATINA a few years ago. This site shows it off nicely:http://www.lingua-latina.dk/index2.htm
The sample pages are quite revealing to the potency of the book:http://www.lingua-latina.dk/samples.htm
Curiously, I also am 23 and a Scorpio — my Latin blog ScorpioMartianus.com
is dedicated to the theme.
As for the differences between Modern and Ancient Greek — they are manifold! I think a topic was started recently to answer that question, and there are experts here in both languages who could inform you better than I.
The main differences:
• grammatical usage
Which, really, could be true of any two languages! But I'll let the others clarify.
Welcome! Vale et vale.