Voltage pulse stimulator for biosensing applications

Hello, I am new to arduino but I would like to make a basic voltage pulse system to test some materials for a biosensor. I need to make a basic low-frequency (~14 hz) pulser with a voltage I can vary from about 5 mv to 500 mv (I don't mind having to reprogram to change this part). I haven't done anything quite like this before, but I have access to several arduino megas and various electronic components (potentiometers, caps and resistors, etc). I know this is vague but if anyone could give me some pointers/instructions or suggest tutorials on how to build this, that would be wonderful! :)

I need to make a basic low-frequency (~14 hz) pulser with a voltage

That part is easy (meaning I can help). At that low of a frequency, blink-without-delay is a good choice... http://www.arduino.cc/en/Tutorial/BlinkWithoutDelay

We'll use micros instead of millis to reduce the error...

const unsigned long INTERVAL = 1000000 / (14 * 2);

unsigned long previousMicros;

void setup() {
  // Other initialization goes here
  previousMicros = micros();
}

void loop()
{
  unsigned long currentMicros = micros();

  if ( currentMicros - previousMicros >= INTERVAL ) {
    // Toggle the output here
    previousMicros = currentMicros;
  }
}

The blink program works for the frequency, but the pins on the arduino can only supply 40mv of current. Unless you want to put 12 of the IO pins in parallel, you'll have to use some transistors, capacitors, and the PWM system.

and the PWM system

Why?

Thanks :) I am currently uploading some code to test out, taking a bit long though, we'll see. I found this website, but do you think it is helpful? http://auctoris.wordpress.com/2011/05/14/arduino-function-generator-part-1/

It would be fine to just have a small voltage (say around 5 mV) pulsed at a frequency that I can vary, either in some method (using potentiometer?) or just modifying delay value and reloading the code each time would be fine too. I'm just a little confused, I tried some different pre-written codes but am having issues, it is taking forever to load. I have some LED's to test on the pin 13 and ground when done... Thank you all so much for your help, I'm just learning and have only rudimentary experience with C, used to doing everything on a breadboard and I'm a newbie. Any other help would be appreciated :)