First off actually know your size. I am 5'8" and am comfortable on a 17" - I think 17 - 18" might be big for you although I think the height limit for mediums are usually 5'6" - 5'10"

Next understand components - do your research. The internet is a wondrous thing! I ride both a hardtail and full suspension bikes... getting older I like the cush and control of the fully but there is nothing like the speed and skill set needed to ride a hardtail. If fast up or down is your thing, a hardtail is suitable. Especially since your price point is so low.

I would actually not consider anything other than a hardtail or rigid. Any full suspension bike you can afford is either going to be crap (Zephyr is correct) or so worned out it will cost a fortune to refurbish. Less moving parts less to worry about that needs replacement or can break. True story, a friend of mind bought a $180 Wally World full suspension bike and was riding some tough trails. The fork pistons actually pulled out of the sleeves and he had a horrific spectacular crash. It's been 4 months; hopefully he can ride again.

JUst make sure whatever you buy fits you and is mechanically sound. Hopefully if and when you get into the sport - you can upgrade.