A very general and unspecific question, isn't it?
Is there anything wrong with using the built-in help and quicksheets?
Unfortunately the Mathcad Users guide (https://www.ptc.com/view?im_dbkey=129422) is no complete documentation anymore (it ends with chapter 13) but if you search the net you will find user guides for older versions (preferably version 11) which are much more comprehensive and include chapter 15 on programming.
Also you may find some tutorials like http://archives.math.utk.edu/ICTCM/VOL16/C029/paper.pdf and there were quite some books written on Mathcad which you may preview at your local book store.
You may also consider installing the E-book/handbook "Inside Mathcad: Programming" http://communities.ptc.com/community/mathcad/blog/2010/05/13/inside-mathcad-series-programming-solving-and-ode-solvers
So study the literature available and if you have a specific question you get stuck at come back here and ask.