The first seal in the Chinese brush painter’s collection is usually a name seal. Once you have chosen your painting name, then you can get it carved into a seal.
A name seal is almost always square, and usually carved in relief – red characters on white ground (yang). They may also be intaglio – white characters on a red ground (yin), though this is less common.
To balance the composition of the seal itself, additional characters can be added to the name, such as “yin” (seal) or “zhi yin” (…’s seal). The characters used on seals are not normal Chinese script, but an older form called seal script.
Name seals can also come in pairs (or sometimes as a double seal) of yin and yang: the yang seal will be above the yin. If you do different sizes of painting, then you will probably need more than one size of name seal (and possibly both yin and yang).
The name seal can be placed anywhere on the painting, but would most often be used after the calligraphy. If the painting is quite dense, then one name seal may be enough.
If you want a personal seal, scroll down here for a list of sources. Be sure to specify the exact characters you want carved.
There are other types of seal for use on a painting, including your studio name and leisure seals.