You have to divide decimal number by 2 and then to write the reminder. Then you have to continue to divide the result by 2 and so on. At the end you have to read all those reminders ( numbers 0 and 1 ) backwards and you will get the binary number. Example: Convert 10 ( decimal ) to binary.
10 : 2 = 5 | 0
5 : 2 = 2 | 1
2 : 2 = 1 | 0
1 : 2 = 0 | 1
↑
Finally: 10 ( decimal ) = 1010 ( binary )