Opening - introduce yourself. If you are applying for a particular position say where you saw the job advertised.

If you are making a speculative application state what it is you require from the company.

Second paragraph - say why you have chosen to apply to this employer. Draw attention to the relevant parts of your CV

Third paragraph - state what you can offer. Highlight your skills and attributes which will have the greatest appeal. Provide evidence of your suitability.

Final paragraph - end on a positive note eg "I look forward to hearing from you".   Say when you would be available for interview.