A cover letter can almost be described as a "hand shake".
Name, address, phone number and email address
Details of the job you are applying for
Where the job was advertised
In a few words what you are qualified to do for the job
If the job is asking about flexible hours or online training mention that you are available and flexible.
Invite the advertiser to contact you and discuss your resume further.
Kind regards and sign off.
Don't redo your resume in your cover letter - that is what your resume is for.
Quite often Resumes and Cover letters get separated so make sure everthing is in your resume.