I would definitely read the article that Richard points to. But if you need a simpler explanation, I hope this helps:
Basically, as you said, there is 1 sign bit, 8 bits for the exponent, and 23 for the fraction. Then using this equation (from Wikipedia )
N = (1 - 2s) * 2^(x-127) * (1 + m*2^-23)
s - , x - ( 127 ), m - , ( ).
, 0xFF . , 0xFE.
,
N = (1 - 2*0) * 2^(254-127) * (1 + (2^23 - 1) * 2^-23)
N = 1 * 2^127 * 1.999999
N = 3.4 x 10^34
, , , -3.4 X 10^34.
Q.E.D.