How can I prove this product? Suppose two numbers $a$ and $b$ such that $a+b=1$.
How can I prove that the product is 
$ab\le \frac{1}{4}$ ?

Depends what you are supposed to use. One simple way is  $$a+b=1 \Rightarrow a = 1-b.$$ Thus, $$ab = (1-b)b = b-b^2.$$ Therefore, the max of $ab$ is attained when $$\frac{d}{db} \left(b-b^2\right) = 1-2b = 0 \Rightarrow b =\frac{1}{2}.$$ Thus, $$\max\{ab\} = \left(1-\frac{1}{2}\right)\cdot\frac{1}{2} = \frac{1}{4}.$$
Edit: To be complete (as asked about in the comments), we can use the "first derivative test" to demonstrate that this is indeed a maximum. In particular, for $b = \frac{1}{2} - \epsilon$ ($\epsilon>0$), $$1 - 2b = 1-2\left(\frac{1}{2} - \epsilon\right) = \epsilon > 0,$$ and similarly for $b = \frac{1}{2} + \epsilon,$ $$1 - 2b = 1-2\left(\frac{1}{2} + \epsilon\right) = -\epsilon < 0.$$ Hence, $b = a = 1/2$ is indeed a maximum.
