OC'ing myGPU first time need some advice please :]

MSI Afterburner 2.2.0 (newest release).
Your preferred Nvidia driver.
Unigine Heaven 3.0 / 3DMark11 (I use both to double check an oc)

Start by going to settings and enable voltage control as well as voltage monitoring. Then click on the monitoring tab and navigate to the Activate hardware monitoring part. Place checkmarks at the things you'd want monitored (and eventually also in the OSD).
When done, close settings.
The OnScreenDisplay is nice to have in order to watch temperatures and memory usage. It's very intuitive to use and setup.

If you want to determine your card's "sweet spot" it's a good idea to write down your core clock, voltage, temperature during benchmark and the achieved Heaven Benchmark score of each succesful run. This way, when you have finally reached the end of your card's overclocking capabilities, you can easily determine what clock speed and voltage is the best of both worlds (temperature and performance). For example:
If you need to add 20mV to go from 840-850mHz, but only needed to add 10mV to get from 820-830mHz, I would stay at 830. (This is a complete bogus example and is not to be considered usable).

Always watch your temperatures when overclocking. I don't like going over 70C when gaming, however this is subjective and up to the user himself to determine.
Now to overclocking:
Note that you have an older card, so the results may not be as incredible as you thought they would be.
Start by adding +30mHz to the core clock, click apply.
Launch Heaven and set the settings as you would in a game (don't go with 8xAA though). Sit back in your comfy chair and watch for artifacts or excessive screen tearing. If you haven't seen an artifact before - don't worry, you will today
So what do you do if artifacts and/or screen tearing occurs?
Close Heaven and open up Afterburner again. At the top you will see a slider called Core Voltage, we're going to be increasing this. The AB Voltage slider increases in steps of +5 - +8 (in my experience using the previous betas). Increase the voltage by one step and click apply. Repeat the Heaven benchmark

If you go through Heaven without artifacting or driver crashing we can proceed.
Since you have an older card we will be taking smaller steps compared to if we were using a newer card.
Add +10mHz to the core clock and rerun the heaven benchmark.

And finally; run your most graphics intensive game for an hour or so to see if it is also stable in a game environment.

This is basically what there is to it. In terms of reaching your max overclock and then finding your sweet spot, it's just about keep trying.

Good luck and have fun with it mate!