You can get your RRSP deduction limit from either your Notice of Assessment (after you submitted last year's tax return) or by logging into CRA My Account.
If you don't have either of those, you can leave this as "zero" or whatever amount has been carried forward by TurboTax and the CRA will adjust your return. Make sure you keep your 2017 Notice of Assessment so you have this value for next year (or signup for CRA My Account if you haven't already).
CRA My Account: http://www.cra-arc.gc.ca/myaccount/