MAC address contains 12 characters, with digits from 0 to 9, and letters from A to F. The letters may be either upper or lower case. Sometimes each pair of characters is separated by a colon, dash or a space. Sometimes there is no space at all.
For an external wireless card, check the card to see if the MAC address is recorded on a label.
If your computer has an internal wireless card, you'll have to use the operating system to find the MAC address.