Do so by clicking on the black Apple icon in the upper-left corner of the screen. It's near the top of the drop-down menu. It's an icon containing a blue cloud on the left side of the window.  If you're signed into your Mac with your Apple ID, your Apple ID will be the email address under your name in the left pane of the window. If you're not signed in, you'll be prompted to enter your Apple ID and password. It's below the password field in the dialog box. It's located near the bottom of the dialog box. It's in the text of the dialog box or enter iforgot.apple.com into your browser. You will need to enter your first name, last name, and email address associated with the Apple ID. You can enter in any prior email addresses as well, but this isn't required.  Click on Next when you've finished filling out the form. There is a very good chance that your Apple ID is your current email address. You will need to enter in your birthday before you can proceed with retrieving the Apple ID. You are given two options for retrieving your Apple ID: you can receive your login information via email, or you can answer a couple security questions and have it displayed in your browser.  If you choose to have the information sent to your email, it will be sent to your current email address as well as any other email addresses you have associated with the account. If you choose to answer security questions, you will be asked two of the questions you set up when you created the ID originally. If you chose to answer the security questions, your Apple ID will be displayed on the next page. You will be asked to create a new password for your Apple ID. If you asked to retrieve your Apple ID via email, you will receive an email with a password reset link. The email address you received the message on is your Apple ID.

TL;DR in Indonesian:
Buka menu Apple. Klik System Preferences. Pilih ikon iCloud. Klik Forgot Apple ID or password? Klik Forgot Apple ID. Klik iforgot.apple.com. Masukkan informasi Anda. Verifikasikan tanggal lahir Anda. Pilih cara yang diinginkan untuk mendapatkan kembali ID apple tersebut. Setel ulang (reset) kata sandi Anda.