if i get below 70 atar should i repeat?
No, that may be enough to get you straight into UWS or ACU depending on what course you want to do.
actually i turned 17 years old in april 2012, so im meant to be in year 11 right now
Doesn't quite work that way, a large portion of people are in the same position.
I'm sorry but i personally believe people shouldn't get another chance at an ATAR unless they have a legitimate reason to repeat. It just seems against the spirit of the HSC to just keep repeating until you get the marks you want.
There are path ways into uni, and any course, no matter what ATAR you get. With a lot of those pathways you wont even lose any time, and would be ahead a year compared with redoing the hsc. I often read on here people saying that if you repeat you'll just get in to the same habits again, make the same mistakes etc. and it often ends up being a waste anyway. Remember, the ATAR is a rank and 70 isn't something you should neccesarily be dissapointed with.
There are three weeks until your HSC now, put in the effort now and you won't even have to worry about repeating or pathways into uni. Ask anybody who crams - three weeks is PLENTY of time. The HSC externals effectively make up 50% of your ATAR and i doubt your internal marks and ranks are as bad as you think they are. Do you really want to have to do all of year 12 again? Good luck!