Very close-up photographs of things is macro photography.
A camera lens is designed to take something very large and make it very small, so the cheapest way to achieve macro photography is to take a camera lens, such as a 50mm, and reverse mount it. An enlarging lens is an even better solution because the working distance (about a foot from the sensor plane to the subject) uses the lens' sweet spot -- the distance that it was designed to excel at.
A bellows is required for serious macro photography. It can do reproduction ratios between about 1:1 to 10:1 and greater. An alternative to a bellows is extension tubes, but these are not as flexible.
Focusing is the most critical part about macro photography. After all the details have been sorted out like setup and lighting, the most time is spent micro focusing the bellows. Depth of field is very small/shallow in macrophotography.
A good bellows mounts on a tripod, allows one to increase/decrease its length to set its magnification, and then allows one to focus by moving the whole apparatus, including camera, away from or nearer to the subject. An inexpensive bellows focuses by moving the lens from the film plane but this is also changing the magnification at the same time making it very difficult to get the magnification and focus where it's wanted.
Also, a good bellows does not allow light through its pleated fabric (accordion) because this will cause a drop in contrast.
Close-up lenses are fine for a simple, quick and inexpensive solution but they are not going to allow as large of an enlargement as a bellows can and the image quality does suffer.
A macro lens is probably the easiest solution and is a good choice for the field. Some produce very good results but they are not designed to enlarge a subject as much as a bellows can. Some macro lenses do focus to as much as 1:1 which means, for example, if taking a picture of a fly then the actual image of the fly projected on to the camera's sensor will be as large as the fly itself, hence one-to-one. Then, it is possible to go beyond 1-to-1 with the addition of a teleconverter such as the Nikon TC-200, TC-14, TC-300, TC-201, TC-14A, TC-14B, TC-301, TC-14E, TC-20E, TC-14E II, TC-17E II, TC-20E II, TC-14E III or TC-20E III. Teleconverters TC-14, TC-300, TC-14B, TC-301, TC-14E, TC-14E II, TC-14E III, TC-17E II, TC-20E, TC-20E II and TC-20E III all have elements that protrude from the teleconverter and can only be mounted on lenses with room to accept this protuding element in the rear mount. The upside is that these teleconverters generally give better results than the ones without protruding elements. If unsure, then buy a TC-14A, TC-200 or TC-201 teleconveter.
Buy a longer macro lens like 105mm or more. The longer focal length will give a greater working distance from the subject.