First off, dont expect much out of that card.
Overclocking is a time consuming process. Start with raising the core clock in steps of 3-5MHz using coolbits, testing for stability between every step.
When testing, keep your eyes open for visual artifacts, or any glitches/weird behavior in the game. After you've found the cards maximum core clock, set the clock 3-5MHz slower than maximum and keep it there. Repeat this process on the memory.
There you have the absolute basics in overclocking a nVidia videocard.