A good tool to learn more about Dynamo would be the Dynamo Primer if you are interested in learning more. In it, it also explains how to retrieve and set parameters of Revit elements. I would strongly recommend reading through it and at least a few tutorials on this website as well (at the top, you can click Learn to find more resources and video tutorials).
As for your question, there are built in GetParameterByName and SetParameterByName nodes that would do exactly what you want. Just feed it an element or list of elements and a string that has the Parameter name (“Mark”). Like all programming, things are case sensitive and capitalization is important.
For ‘if the sheet no. is null’, a simple if statement would work. Try out what you can on your own and if you get stuck, come back and we can help you work through it.