A CV is quite simply an 'advert' to sell yourself to an employer. You should send a CV to an employer when they ask for one in a job advert, or when you are enquiring if any jobs are available. So the purpose of your CV is to make you attractive, interesting, worth considering to the company and so receive a job interview.
Your CV is your best chance to convince a prospective employer that you are the best person for the position. Because the employer may have several hundred enquiries for the position he/she will only choose a few suitable people for an interview. So it is vital that your CV paints a clear picture of you and that it highlights all aspects of your life and career that is of interested to any future employer.

Bear in mind when constructing your CV that the decision maker/potential employer will probably only spend 30-60 seconds reading your treasured life history - which is not much time for you to market yourself. A CV is designed to get you an interview and a foot in the door, it is not to get you the job.

Where to start
Sit down with a piece of paper. Look at the job(s) that you are applying for. Consider how your skills, education, and experience compare with the skills that the job requires. How much information do you have about the job description?
Sometimes employers do not give enough information. Ask for more detail if needed. Spend time researching detail about the job(s) that interest you and information about the employer - their structure, products, successes, and approach - from:

Their own publicity, reports and publications
A library (business reports, trade papers)
College career office
Newspaper reports
The Internet

Gathering and organising the facts
Start working on your CV by collecting and reviewing information about yourself: Previous positions, job duties, volunteer work, skills, accomplishments, education, and activities. These are the raw materials of your CV. This is also a good time to review your career goals and to think about which past jobs you have liked, and why.

After compiling this information, research the occupations that interest you. Determine what duties they entail, what credentials they require, and what skills they use. Your CV will use your autobiographical information to show that you meet an occupation’s requirements. Remember: Even if you do not have many specialised and technical skills, most occupations also require abilities like reliability, teamwork, and communication.