Work through the problems on this PowerPoint with your child to help them to understand the mathematics function of subtraction. This is a good resource to use for an introduction to subtraction!
You may want to do some practical subtraction with your child first before looking at the PowerPoint and then use objects alongside the PowerPoint.
In school we call 'sums' number sentences. Just like we read and write left to right we do simple addition and subtraction left to write.