How do I sign into my iTunes account?

I have spent hours today trying to find a way to sign into my iTunes account, which I havn't used for many years. I can see it in my apps but there is no where that will let me sign into it.... i'm losing all hope of ever renewing my music ...I am definitely NOT a PC expert, infact I'm a bit of a dunce when it comes to tech so if you can help, may I ask you to use plain English please or I won't be able to follow instructions....many thanks all :)

[Re-Titled by Moderator]