How do we use myself as the only subject of a sentence?
For example I once heard some people saying Myself am to be blamed. Is this grammatically correct? How is it different from I am to be blamed?
No, it's not grammatically correct, which is how it's different from
I am to be blamed., which is. You might use it if you heard that the news media were going to publish a story blaming you. But it you wanted to state something was your fault, you wouldn't say that, you'd say blame me or it's my fault.
I myself am to be blamed is grammatically correct too, and means the same as I am to be blamed (by the media etc.) but adds emphasis that you are to be blamed rather than someone else perhaps more deserving of blame.
Myself, I prefer driving to cycling is also grammatically correct, and again, the addition of myself adds a certain emphasis: rather than a detatched fact about your preference, it suggests comparison, in response to another person's preference just mentioned.