This is your basic op-amp non-inverting amplifier
Where gain Av = 1+R2/R1
You want a signal which at maximum is 0.21V to be at maximum 5v. So that means you need a gain of 5/0.21 = 23.8, which is easily achieveable with an op-amp.
This can be made using standard resistor values of:
R1 = 10k
R2 = 220k
which would give a gain of 23.
As long as the op-amp is capable of reaching its supply rails, that would work. However, generally they are not - so a 5v supply would not yeild a maximum output of 5v.
You could combat this by using amplifying the signal to say 3.3V and then setting AREF to 3.3V.
That would need a gain of 15.7 which can be achieved with:
R1 = 10k
R2 = 150k
Edit:
Looks like others beat me to it.