This wikiHow teaches you how to sign in to iTunes with your Apple ID. You can do this on both desktop and mobile versions of iTunes, although you'll need to create an Apple ID if you don't already have one.
Steps
Method 1 of 2: On a desktop

Step #1. Open iTunes
Click or double-click the iTunes app icon, which resembles a multicolored musical note on a white background.

Step 2. Click on Account
It's at the top of the iTunes window (Windows) or screen (Mac). A drop-down menu will appear.

Step 3. Click Login…
This option is in the middle of the drop-down menu. A pop-up window will appear.
- If another Apple ID is signed in, first click Sign out and then To register.

Step 4. Enter your Apple ID email address
Type your Apple ID email address in the "Apple ID" text box at the top of the pop-up window.

Step 5. Enter your Apple ID password
Type the password you use to sign in to your Apple ID account in the "Password" text box in the pop-up window.

Step 6. Click Login
This is at the bottom of the window. This will sign you in to your Apple ID account.
All iTunes purchases made with this account will be available in iTunes once the sync is complete
Method 2 of 2: On a mobile

Step 1. Open the

settings of your iPhone.
Tap the Settings app icon, which looks like a gray box with gears on it.

Step #2. Tap Sign in to your iPhone
This is at the top of the screen.
If you see your name and a photo here, you're already signed in to your Apple ID on your iPhone

Step 3. Enter your Apple ID email address
Tap the "Apple ID" text box, then type your Apple ID email address.

Step #4. Tap on Next
This is located in the top right corner of the screen.

Step 5. Enter your Apple ID password
Tap the "Password" text box, then type the password you use to sign in to your Apple ID account.

Step 6. Tap on Next
This is located in the top right corner of the screen.

Step 7. Enter your iPhone passcode when prompted
Enter the passcode you use to unlock your iPhone. This will confirm your registration and add the iTunes content from your account to your iTunes library.