COMPARATOR help

Hi I'm sort of new to electronics and would like help on something..
How would i use a comparator, so that if the input voltage is lower than say 3v for example, then it outputs high. If the input is higher than 3v then it outputs low.
Any help/websites/schematics is welcome and thankyou in advanced

sonny

The comparator (or op-amp) have +input and -input. When the voltage on +in higher as -in, the output go high.
In Your example, connect +3V to +in as reference, and connect the measured voltage to -input.

Try this circuit:
You can use a potential divider instead of zener-diode for exact threshold setting because 3V is not a standard value. You'll get 3.3V

Also, LM324 is not a rail-to-rail o/p op-amp, so you won't get exactly 5V at the output. It will be less than 4V. To get exact 5V, adjust your power supply accordingly.