Thank you friends for clearing my doubts and responding so quickly. Thanks again.

The first one is a question. It is similar to "What did I do?" The second is a statement, not a question (not a correct one at least). You would use this statement to describe what actions you have done. "What I have done is I have made breakfast." However, this phrase is not used commonly. An easier way to get to what you are saying is simply "I have made breakfast."

Although the first one is more correct both are OK. In Terms of differences, they are more stylistic and obvious in verbal speech: e.g. What HAVE I done? (AAAhhhhhhh! I have been foolish, how could I make a mistake like this?) e.g. What I have done? You want to know what I have done? I'll tell you what I have done. I have blah blah blah... Hope the above has been of help.

the meaning is the same, the only difference the context in which you use them; formal and infomal

But what about their meaning. Are they used in same reference or meaning? Thank you