(You would use "t'was" or "'twas". People more commonly spell the word "t'was" as "'twas", but it is more accurate to say t'was according to the rules of contraction in English grammar.)
Jill: T'would be cool. T'would be nice if you'd give me a call later this week to remind me though!
James: Would it? T'would be very difficult, since I don't have your number!