New to Sage 50 - Need help bringing over all old accounts from Simply Accounting

Hello, 

I started a job on July 20th and was told that everything would be switched over to Sage 50 during my interview (she was using Simply Accounting 2012 and also entering everything on excel) and I got here on my 1st day and she was still using Simply Accounting :( 

So that wouldn't have been to bad to just bring everything over when I downloaded Sage, but our accountant has suggested that we do not import everything because there were quite a few incorrect entries from 2012 that we can not clear, therefore her bank balance is not correct and I can't get it to balance on Simply. She uses Excel to balance her bank statement. 

So I am wondering what is the best way to go about starting to enter everything into Sage? I have to do April, May, June, July and will have to do August. She never entered in her bank balance or anything (it's always overdrawn in Simply and she just uses Exel) so I am wondering how to even go about doing that?

This is all new to me so I don't even know where to begin :( except I have already set up my business and entered in all my vendors. 

Any help would be so appreciated :)