When looking for work, there is often an amount of competition around the jobs that you are applying for and if you cannot show why you are the better candidate then you will most likely miss out on the job.
The first thing a potential employer usually sees is your CV and covering letter. They can often very quickly make a judgement on the type of person you are just from this so it is vital that you portray yourself in the right way. There are a number of good CV templates available online that you can use to help you get started with a CV. If you have had gaps in your employment then this may be obvious from the dates of other jobs. You need to think about the reasons why you were unemployed during that time and have an answer ready should you get called in for an interview.
Your covering letter should be personal to the job you are applying for. It should mention the position that you want to be employed in and talk briefly about why you think you would be suitable for the job. You can make reference to your CV but a covering letter should be no more than 3 or 4 paragraphs long and give the employer a reason as to why they may want to ask you in for an interview.