First you need to know the frequency range you need to measure, then state your accuracy and resolution.
Generally there are two modes of frequency measurements in real world:
1. High frequency measurement where you make accurate time window of 0.1 sec say(depending on resolution) and count the number of incomming pulses(frequency of signal to be measured) in this time window. doing some maths you work out the frequency.
2. Low frequency measurement where you make the incomming frequency pulse as timing window and you generate fast pulses internally with known frequency, count these pulses and work out the frequency mathematically. In other words you measure the period of incomming low frequency signal.
Of course you need to convert incomming signal into TTL levels for the micro to handle them.
You also have to do averaging on input signals by adding (say)4 measurements together and divide by 4 by right shifting twice before you display them. In this case you get a nice stable display of frequency.
Mahmood