1) A proof of your citizenship
You must be able to demonstrate that you're a citizen of the country. Residents without citizenship cannot apply for a passport.
2) Application form and fee
In most cases, you can visit the website of your country's passport office and get the forms from there. You'll be able to print and fill them at home.
Then, you can send these forms by post to be processed. Below are links to US, UK and Australian passport offices with information on where to get the application forms.
3) Minors must be accompanied by their parents
If you're an adult - you're on your own. However, when applying for a passport for a minor, both parents should be present (and the minor too).
4) A valid ID document
To show that you're who you're claiming you are, you should typically bring a photo ID with you.
5) Two new passport photos
In any case, when applying for a passport, you'll need a pair of identical, recent passport photos.
You can save money by making them yourself using Passport Photo.
Before you take your photos, spend a few minutes reading our recommendations for taking good passport photos.
Then, use Passport Photo to print them at home, or in any shop - just like regular photos.