Depending on how formal your email needs to be, you may want to start with dear as you would a regular letter. In most circumstances though simply addressing it directly to the person is fine. In a conservative industry you should probably start using a formal greeting including Miss, Mr., Mrs. or Ms. In a more laid back environment though, using just a first name is fine. This sets the tone as being direct but still says that you are a peer of the person to whom you are writing. That being said, if you are addressing your email to a executive officer, such as the CEO of a company, formality is probably more appropriate.