For example, everybody has a personal tax allowance that enables them to earn so much before their salary becomes taxable. The tax code for 2011 for a basic personal allowance is 747L. You should replace the final letter with a five if you want to know exactly how much you can earn before you are eligible to pay tax, so in this instance it is £7,475.
If you have two jobs or pensions, it is likely that all of your second income will be taxed because you will have used up your tax allowance on your first.
Some people will be able to earn more than this before they are eligible for tax because things like special clothing can be taken into consideration. You can check with your local tax office if you think that this applies to you.
If you are not sure what your tax code is, you will be able to find it on your P60 or P45. A P60 is the document that your employer is legally obliged to give you each year; besides your tax code it will also show you exactly what you have earned for that period, as well as the tax that you have paid, your National Insurance number and any other financial matters that are relevant to you, such as student loan payments, statutory sick pay and pension contributions.
If you have left your place of employment, your employer is obliged to give you a P45. This will have the same information as a P60, but will only give your payment details up to the date that you left their employment. It is important that you keep this document so that your new employer can make sure that you are being taxed at the right level.