The information HMRC asks for from your PAYE job comes from the P45 or P60 form you received from your employer. If you do not have one, you should request it from your employer. It is their responsibility to send it to you. You may also receive a document called a P11D where your employer will detail any expenses or benefits you received and need to be reported to HMRC.
Let’s complicate matters even more
Let’s say you get paid by multiple PAYE jobs throughout the year, have freelance income, made some money from selling stock and have two rental properties.
Firstly, you sound busy!
Secondly, you will need to fill out a bunch of forms: the SA100 along with supplementary pages, the SA102 for your PAYE jobs (don’t forget your P11D for any benefits or expenses), SA103 (s or f) for your freelance gigs, two SA105 forms for your properties, and an SA108 for each Capital Gain.
For more on HMRC forms, you can visit HMRC’s website which goes into more detail.