The typical steps to begin researching Jewish families are the same as with every other ethnic group or faith in the United States. Start with the living generation and work backwards, consult with extended family, and gather civil and public records first.
“Ashkenazi” refer to Jews of German or Central and East European origins. “Sephardic” includes those from Spain, Portugal, or countries surrounding the Mediterranean Sea including North Africa. Worldwide, it’s estimated that 75% of Jews are Ashkenazi, 20% are Sephardic, and the remaining 5% are split amongst Mizrahi, Beth Israel or Ethiopian, Yemenite, or Asian Jewish communities. Most Jews in the United States descend from Ashkenazi arriving post-1880, but the earliest Jewish settlers in 1654 New Amsterdam were Sephardic.
JewishGen can help you get started with tutorials, FAQs, and online classes. See:
JewishGen Frequently Asked Questions (FAQs)
JewishGen Online Genealogy Classes
Join and post an introduction on the JewishGen Discussion Group or Search the JewishGen Discussion Group Archives
A Note About DNA:
You may have arrived here via. a DNA test that revealed Jewish ancestry. Your best approach to get started is to contact near matches that share the same DNA, as they may know exactly whom on your tree was Jewish. A good resource to learn more about interpreting results is Your DNA Guide. Know that with Jewish DNA, endogamy is possible and can make matches appear closer than they really are.