A decimal number in binary, octal and hexadecimal.
Divide the number by 2 repeatedly, noting each remainder, then read the remainders from last to first. For 13: 13/2 gives 6 remainder 1, 6/2 gives 3 remainder 0, 3/2 gives 1 remainder 1, 1/2 gives 0 remainder 1 — so 1101. The same method with a different divisor works for any base.
Because one hexadecimal digit corresponds exactly to four bits, so a byte is precisely two digits. That makes long binary strings readable without losing the connection to the bits: 11111111 is FF. Decimal cannot do this, since 10 is not a power of two.
Hexadecimal needs sixteen digits but only ten numerals exist, so the remaining six are letters: A is 10, B is 11, C is 12, D is 13, E is 14 and F is 15. Case is irrelevant — 1a and 1A are the same number.
Multiply each digit by a power of two according to its position, counting from zero at the right, then add the results. For 1101 that is 1×8 + 1×4 + 0×2 + 1×1 = 13. The same positional logic works in any base, with a different base for the powers.