If I worked 30 hours and I make 8 dollars and hour. How much will my check be if after taxes are taken out?

Go to www.paycheckcity.com for a good, free tax calculator.

Run the numbers here -> http://www.PayCheckCity.com

Note that while you might be exempt from federal withholding you probably are not exempt from MO withholding.

let's hope you are eligible to claim exempt on your W-4
but whatever your gross is, rate x number of hours, 7.65% will be withheld for FICA, if Mo has a state employee tax it might be 1% or more or less again on your gross
having claimed exempt no income taxes will be withheld
it is time to learn how to do the calculation yourself, do the math

If you put "exempt" on there, you'll have around 10% withheld, between Federal and State payroll taxes (FICA, FUTA, SUTA, and possibly state income).