Use any shape you like! Be careful when grouping objects as sometimes they will combine/merge weird in unexpected ways.

Here's when you can play with getting your sizing correct and playing with the combine functions some more. Remember, circles and squares can be used as cutting tools too! Play around with the Combine tools.

By now, you're confident in adding shapes, combining and merging. Add your text, play with the font and spacing settings, and remember there's a “merge” button on the text interface for once you squash the digits together. 

Have fun making stuff!!