To order information (documents) from the National Archives go to their home page at www.nara.gov and at the bottom of the home page, click on "Contact NARA." This will direct you to their Contact Page. Near the top, right, there is a link for their Inquire Form. Fill out the forms there including your contact information. After you click "Review your entries" you can submit your request. They explain how and when they will reply at the bottom of that page as well.
They don't have all KTBs but they do have most of them. See: http://www.archives.gov/publications/how_to_order_microfilm.html for more details, their current price and expected delivery information. You need the roll number for each roll of microfilm you are going to order. If you don't already have it, go through their "Inquire Form" above and they will send you the number(s) you need.
KTBs come on a roll of microfilm, not in printed form. You will have to find a viewer that can handle the microfilm. I found one here at the local library (main branch). In the U.S. the same viewers that are used to view census data will work as the census data comes on the same size roll.
The KTBs were turned in to U-boat Command at the end of each patrol so there may be several for a given boat. There are also usually KTBs from a few other U-boats on the same roll. Of course, the KTB for the patrol on which a U-boat was sunk will not be there as it is still in the U-boat. They also have some "reconstructed" KTBs that were created from the radio traffic of the patrol from which the U-boat didn't return. Be sure to specifically request the reconstructed KTB if you want it. They may or may not be on the same roll with the rest of the KTBs for that U-boat. Also note that other parts of the German navy (for example U-boat Command) kept KTBs of their own activities and those are also available. I think they were kept at the flotilla level too. If you need to know exactly what they have use their "Inquire Form" described above to find out.
Also note that they are not perfect copies. Some mistakes were made when they were microfilmed after the war. Some text is chopped off, some is so light or dark it is very hard to read. In a lot of cases, the e's and a's are filled in and look like o's. Sometimes there is bleed-through from the next page too.
For each KTB in addition to the commander's comments there may be portions of a hand drawn map for the patrol, a detailed map of a given attack (when the target was sunk by gunfire) showing every course correction, and sometimes, torpedo reports (Schußmeldung reports). A torpedo report contains the info (torpedo type, detonator type, inputs to the torpedo firing computer, etc. plus a hand drawn sketch showing the position of the U-boat and the position of the target, etc) for each torpedo attack. If multiple torpedoes were fired at the same time (a spread/salvo) they will all be on the same torpedo report. The torpedo reports are not always there. See German Sources - Bibliothek für Zeitgeschichte below for the possible location of missing torpedo reports.
U-boat KTB's are fascinating to read however, they are in German and just knowing the German language is not enough to translate them correctly. The translator needs to be familiar with U-boat terminology and abbreviations and especially their equivalents in your language. There are abbreviations, acronyms, etc galore. For example, you may find ATOS, ETOS, IWO, FT, UZO, BOLD, etc. Another example: "Schuß. Vg 12 E 400, Tiefe 3, Lage 90" translates to "Torpedo fired, target speed 12 knots, distance to the target 400 meters, torpedo depth set for 3 meters, angle on the bow 90 degrees" in English.
See http://www.uboatarchive.net/ - "About the Records" for info on the records and photos in the National Archives. This is also a great U-boat web site.
For photos you must contact them via postal mail at:
National Archives @ College Park
NWCS (Stills), Room 5360
8601 Adelphi Road
College Park, MD 20740-6001
Include your return postal address and any information that may aid them with your search. Note that unlike some other archives you must go in person or have someone go for you for photos. They won't copy them and send you the copy. You can go and scan them yourself but you must bring your own scanner.
On-line photos of U.S. Navy ships from WWII (National Archives):
Web site: http://www.navsource.org/archives/home.html