How do you get the year of birth or death of a ancestor?

Question:

How do you get the year of birth or death of a ancestor?

to get my grandfather birth certificate I need to ha his date of birth, and the year of death.

Best Answer - Chosen by Asker: How do you eat an elephant? One bite at a time. Seriously.
Research is broken down into small steps, and every step of the way is unique to each one of your ancestors. One of the beginning steps is to work backwards. Your grandfather's death is more recent than his birth, so you go for that first.

If you don't have access to grandpa yet, kick back down to the parents place and date of birth. Make a chart that includes the siblings (this is always important). If your parent is the eldest, then you can figure that the marriage probably was 1-2 yrs prior. If parent is the youngest, then looking at the eldest child will help. Are all the children born in the same place? Look for a pattern of migration.

If your grandfather (or even parents) died in the last 50 or so years, in the US, he should be in the social security death index. That will include his date of death and his date of birth, as well as where he was living when he got his social security card. Other death sources can be his tombstone, family photos, old letters or documents. mortuary records, etc. Also his death certificate itself, which should include the names of his parents.

If he was born prior to 1930, he will also be somewhere in census records (in the US). If he was born before 1900, it is not likely he will have a birth certificate. Those are vital records that were established, state by state. When you know the state and year of his birth, there are locations online that have charts for that information. Finding him living with his parents in census records is going to tell you things not only about him, but other family members. Every census taken had different requirements, which show up as you go along. The usual is names of persons in the house, their age, sometimes month and year of birth, if they owned land (another valuable record), and since 1880, it will also give the birth place of the parents.
Prior to 1900, the sources will change. Census is more important, cemetery records, wills, biographies in local history books. and more. The census will change prior to 1850, as to what it offers.
Not everything is out there online, but once you start, there really is a lot (and no, not all for a fee). Every county in the US has a genweb site, where volunteers collect the kind of things that will help you. Rootsweb has family files that other researchers have collected and posted, with varying degrees of accuracy. EXPECT contradictions in records. This is when you compare notes as to where the data comes from, and which fact is more logical or "proven". There are message boards and email lists that deal only with one surname. People looking for the same persons or families that you are.
It is impossible to say exactly where your grandfather or other ancestors records are. In order to find it, you need to know the location, the time frame, and what was available in that specific time and place. The operative word here is that it is not possible to give a "general" answer. You build on what you have, and each new step points to the next. But that is exactly what makes it such a challenge!!