i want to start riding.
i really think that its a sport i could get into to.
i dont want to spend much on a bike until i know im into it.
Sounds like you've never ridden a road bike. Do you know how to ride any bike? Ever used gears before?
dirt cheap road bike
tiny bit too small,
from a sports store.
Sounds like you haven't done enough research into buying your first road bike.
i not buy i road bike if i am not acostumed to riding one.
back pain would discourage me from riding.
a beginner.
i didnt like his answer very much.
Sounds like you pretty much told the sports store sales guy you're a complete noob and he's just trying to help you out. I'm sure he could have taken your excitement and probably talked you into buying a nicer new bike that may have just been out of your price range that you'd probably scrape up $ for (did you even think about jersey, shorts, helmet, water bottles, pump, tubes, clipless pedals, shoes, etc? They add up and suddenly your $750 "cheap" road bike costs less than all the ancillary gear you bought.
if im new to the sport should i start with an all purpose?
Probably not but I'd start out with at least riding some bikes. Tell us more about you - how old are you? Are you athletic? Do you just want to ride on the weekends or daily to work/school?
Sounds like you should talk to the guys and gals in your local club that you want to ride with. They'll give you good advice and let you try some of their bikes. I'd be willing to bet that at least one of them has a great, maintained, used bike that would be better than anything new you can buy with the $ you have to spend to try out a sport you don't even know you'd like.