Computers store all data as binary, numbers are easily stored this way.
Humans work with a base 10 numbering system (we have 10 fingers), however computers can only store data using electricity (on and off) which is base 2.
This article explains binary in a simple way
128 64 32 16 8 4 2 1 are the headings at the top of the binary numbers so 10001000 would be 128+8 which is 136
For you exams you will be required to show the working for binary conversion, however, you can check this by using a calculator that has a binary function. The calculator below was found in a local pound shop, you can check if your calculator has binary by checking the buttons for BIN, OCT, DEC, HEX.
You can make a little Binary Calculator with the attached file. The photographs show it in action, simply fold the flaps to make the binary number and add the numbers shown. (teachers note that for 2’s complement just change +128 to -128)