For this you use a representation that is referred to as binary system...

You can represent with this 4bit system all numbers from 0 to 16. An example...
1011 = 1 × 2^{3} + 0 × 2^{2} + 1 × 2^{1} + 1 × 2^{0}
= 1 × 8 + 0 × 4 + 1 × 2 + 1 × 1
= 11
This is known as a unsigned binary system. Later, you get to know different systems, as there are more systems to represent numbers.
The last digit of the binary number (2^{0}) is called LSB (Least Significant Bit), the first digit (2^{3}) als is called MSB (Most Significant Bit).
How do you add two binary numbers...?