If you are going to stay out of trouble, you need a source of income and therefore you need a job.  

The first thing you should do on leaving prison is take your discharge papers including the tax documents, to the Job Centre.  Here you can sign on as unemployed.  Unfortunately you do not get your National Insurance Stamp credited whilst inside and as your ability to sign on depends upon NI credits it is likely that you might no be able to claim depending upon the rules at the time.  Also age and savings or other income can affect your ability to get money.  However, whatever your financial circumstances, if you are between 18 and 64 you will get your National Insurance stamp purchased for you.  Known as NI Credits.  Also the Job Centre is a good place to find work and some staff can actually be helpful.

Newspapers, personal approaches are all good ways to find a job.

Do not believe that you are a with no hope of a future.   There are jobs that are suitable for your experience and qualifications.  If you have been in prison for any length of time you will have gained qualifications to demonstrate to a prospective employer that you have skills.  Hopefully prison will have given you the skills to write your own CV to accompany your letter of application.  If not, ask at the Job Centre for an appropriate short course.

You will need to be honest with the Job Centre about your past life.  They will know anyway from your discharge papers.  Whether you should volunteer the information to a prospective employer is for you to decide.  However, if asked, you should give details of your offence and say why you think you are a different person and will not commit an offence in future.  An employer should not discriminate against you but if you don't get the job you will leave believing that your record is the reason for not getting the job.  Also be sensible, if you are a convicted pedophile  don't apply for a job in a children's home and if you are a bank robber don't apply for a job in a bank!

The Resettlement of Offenders legislation means that after a set period of between 5 and 10 years most convicted offenders can legally say they have no conviction.