It depends on where you are, the rules and procedures are very much localized. Where I am you show up at the Public Health Department, give them a name and birth date, pay a fee, and wait around for an hour and poof, birth certificate (assuming you were born here). You can even get someone else’s birth certificate.
There are instructions for each state posted on the Internet. Look for your state’s department of registrations, or some such category – vital statistics, births and deaths, etc.
I had to send away to the town where I was born recently to request this. You fill out the form with personal details (parents, birth date, mother’s maiden name, etc.) and the city / county clerk will look up to verify and give you the BC.
I’m sure there are procedures in place in case you don’t know some vital information; talk to your city or county clerk (city or county of birth).
If U.K resident you need to know which town/city you were born in. Go to the registry office in that town. Give your name,date of birth along with the hospital born at. They will need names & birth dates of both parents if possible – you will either get a copy there & then for a fee, or make an appointment to collect the day after
I just went down to the Vital Records Office (covers births, deaths & marriages) and requested one when I needed a replacement for my daughter. Since she was born in this state, it was just matter of paying a small fee & I walked out with a copy 10 minutes later.
When I needed a replacement of mine, I was able to order it from the Texas Vital Records office online. I’m glad I didn’t need it right away.
If you live in the United States, obtaining a legal birth certificate will require some work on your part. first, i have to ask why you do not have one? you did not say. second, if you have a family bible with your name and date of birth in it, this most likely will be accepted by your state as evidence needed for your birth certificate.