Order of acquisition in typically-developing children is based on how frequently they hear the different question forms:

http://www.ncbi.nlm.nih.gov/pubmed/14513470

That means, each child will have a different order that will work best for them based on which question forms they have heard most often.

• Yes/No is typically a high-frequency question type so that is a good place to start.  There are also only two answers so that also makes it a good choice for the first to approach.
• What, Who, and Where are often the next to develop as they have more concrete answers than the other question forms.  I would suggest trying a bit of each and go with whichever one the child has the most success with
• Why develops after the child begins to understand cause and effect.  Make sure that is in place before attempting
• When develops after the child understands basic time concepts
• How develops in relation to each type of how question (procedural how questions once they understand steps in a sequence, describing how questions once they understand descriptors)