How to log in to iTunes?

What hardware are you using?

"When you sign in with your Apple ID on your iPhone, you will be signed in to the iTunes Store, App Store, and Apple Music automatically so that you can make purchases or begin using Apple Music quickly and easily."

From: Apple ID & Privacy - Legal - Apple Account & Privacy- Apple

It will be the similar for other hardware.

If you are using a different account for iTunes purchases, you may need to check what is says under Media&Purchases. In the support article: If you subscribe to Apple One and use a different Apple Account for iCloud - Apple Support Apple alludes to using a different Apple ID just for media, subscriptions, and purchases, but doesn't describe how to do this. See if these work:

In iOS/iPadOS: Go to Settings > [your Apple ID] > Media & Purchases. Sign out of the current ID and sign in with a different one.

In Ventura or later: First, quit Settings. Then, in App Store, choose “Sign Out” in the Store menu, then click on the “sign in” button that appears in the main window. You can then sign into the other account. (from: Using different Apple ID for media & purc… - Apple Community )

Related to that is: Use a different Apple ID to share purchases with Family Sharing as outlined in Use a different Apple Account to share purchases with Family Sharing - Apple Support

Read this support article to learn how to sign in with an Apple Account on various kinds of devices --> Sign in to your Apple Account on the web or in your device settings - Apple Support

To do what exactly?

On what device or computer?

Are you referring to the iTunes application? the iTunes Store? iTunes Match? maybe the Music App? something else?

Please provide details and context and someone may be able to advise.