I'm new to Arduino and programming so I'm a bit overwhelmed by the volume and variety of stuff here haha.
For a project I'm looking for the necessary sensors/opamps needed to trigger frequency dependant actions. So say a low frequency of say 90hz picked up by the mic sensor would trigger x while a higher frequency of say 12k would trigger y.
The majority of microphones I found are just voltage dependant (=volume) and not frequency based. I have the explore IoT Kit.
Also are there any codes regarding this type of project I could use as a basis?
It is not the microphone it is the circuit after the amplifier. A module that gives just the peak amplitude level is followed by an "envelope detector" circuit, a typical one is shown here. https://en.wikipedia.org/wiki/Envelope_detector.
Most microphone modules I have seen have both, but you can always modify any board by connecting direct to the amplifier's output, or by removing the envelope detector circuit.
To do the audio processing on the Arduino in software, you're going to have to tackle two problems:
(1) capture a sample of sound using a microphone input -- maybe a MEMS direct digital microphone
...or an analogue microphone with an audio sampling program. Google found me some examples.
(2) Look for your target frequencies. There are several ways to do this. There are Arduino FFT and FHT libraries http://wiki.openmusiclabs.com/wiki/ArduinoFHT
Look into the Goertzel algorithm, which is a good deal faster (maybe not using floating point like this example) that allows you to measure the audio power just at specific target frequencies:
All of this is possible on a 16MHz AVR. You're not going to get continuous processing, but sampling of 20ms samples every 200ms or something on that order.