To understand the information you discover and to make it easier to find, you'll need to learn about military structure. Doing so will save you hours of wasted time looking in the wrong places. In addition, you'll need to study the history of the Civil War–its causes, its major battles, its results. Go to The American Civil War Homepage at http://sunsite.utk.edu/civil-war/warweb.html And while you're studying the war, have a good map of the United States east of the Mississippi handy to pinpoint the geographic location of the major actions.
From what you learn about the War and what you know about your ancestor, draw up a profile, no matter how basic. If he were in the infantry, you wouldn't search the cavalry or naval unit rosters and so on.
Begin your profile with your ancestor's name and possible nickname. Do you know if he served as an officer or a regular soldier? In which State did he enlist? What years did he serve? Which type of unit did he serve in—artillery, cavalry, infantry? Was he killed in action? Do you have a photo of him? Do you have a copy of his discharge paper or a framed discharge certificate?
What clues can you glean from this profile?
War photography came into its own during the Civil War. Not only did traveling photographers take photos of the battlefields, they would also set up small tents on the edge of Union encampments where they produced tintypes of individual solders which they could send home to their families. These usually had the soldier's name, rank, and date taken, and the unit with which he served. If you possess such a photo, look at it carefully. What type of uniform is your ancestor wearing? Ordinary Union soldiers wore a small cap while officers wore a wide-brimmed hat. What weapons is he carrying? Cavalrymen carried pistols and had a saber at their side while infantrymen carried long rifles with bayonets attached. Does he have any extra bars, stars or braids on his uniform? Each additional one indicates a higher rank.
Each state in the Union had its own fighting militia units. Knowing which state and the number and type of unit will help your search immensely.
Once you've gathered this basic information it's time to begin searching the Internet's Civil War sites to see what else you can find out about your ancestor. Go to the general Civil War sites first to learn the basic command structure, unit histories, and how to read Civil War troop rosters.
The Internet offers lots of Civil War Information sites today, most created by Civil War re-enactment groups who need to know detailed information about their unit's history, including famous battles in which the unit participated. They not only dress in authentic uniforms and carrying authentic equipment, but outfit themselves in minute detail. Plus they not only re-enact the battles but become individual soldiers with a history of their own. Sometimes, the soldier re-enactor represents is a distant relative.
One of the best sites used to be the Civil War Roster at Civil War Rosters. Unfortunately, many of the links on this extensive site were once part of the Geocities Web site network which Yahoo closed down in October 2009. However, while quite a few of the links are now dead ends, others do go to viable Civil War sites. Once on the site, click on the state from which your ancestor hailed. Then look for his unit from that state. If you're lucky, you'll be able to go to a site which has a history of his regiment and perhaps a list of those who served in it.
To find out more information about your Union soldier or sailor ancestor, you'll have to search the muster rolls. Do a search for "Civil War Muster Rolls" on Google. The result is a long list of sites containing muster rolls from various units. Click on any that you see listed that refer to your ancestor's unit, then search for his name. Once you find it, notice if there are any remarks after his name. Here's where you'll find out whether your soldier was wounded or killed in action and on what day. Then check the list of battles his unit fought in until you find the battle that corresponds to the date behind his name.
If he died on the battlefield, you'll want to locate his grave. Usually, the army buried soldiers killed in a particular battle on that battlefield or in local graveyards since there was no way of preserving bodies transported over long distances. Your next step is to search for "Exact Civil War Graves." But actually finding your ancestor's grave may take quite a while. So be prepared.
Before beginning your Internet search, study every clue you can obtain from the material you have on hand. Of the over 4 million soldiers who fought in the Civil War, over 600,000 became casualties. Records weren't kept as well as they are today. And battlefields, though preserved, may not have been where a soldier finally died.
Also, Civil War regiments weren't integrated as they are today. Each ethnic and racial group had its own regiment. Immigrants, Native Americans, and free men of color all fought together in their own groups.
And don't forget to search the Civil War CSA State Pension Rolls. You'll find most of each State's rolls online at Civil War sites. By searching these rolls, you're assuming your ancestor lived long enough to collect his pension. Many soldiers headed West after the Civil War, not knowing that pensions were even available.
The National Park Service hosts several excellent sites for Civil War research, including
The Civil War Soldiers and Sailors Page at Civil War Soldiers and Sailors System Name Search and The Civil War: 150 Years anniversary page at http://www.nps.gov/features/waso/cw150th/.
One of the best sites on which to search military genealogy is http://geneasearch.com/military.htm. Here, you'll find a variety of links for researching the Civil War.
Above all, it pays to be patient and persistent. You'll most likely find the information you need if you look long and hard enough.