My logo consists of letters in black, grey (109,109,109) and pink (244,0,128).
What are some guidelines or techniques that I can use to choose an appropriate secondary color to use on my website, in Powerpoints, and other branded materials?
This also depends on what your logo is for...most of the times colors in logos, websites, presentations are chosen specifically to express something.
They are pretty good at generating adjacent and complementary colors that you might draw inspiration from when in need [they can give you other sets of colors too, I didn't have time now to give all the examples and settings you can make]:
They can also generate examples using those same colors in artwork and websites so you get the idea:
If you are using Adobe Illustrator then there is another way how to pick up complimentary, monochromatic, analogous etc. colours. Simply select or choose one colour and then activate the "color guide" panel. In this panel in the dropdown you will have a big choice of colour variations, select one and test it. :)
You can create a color scheme according to basic design principle of color theory using Adobe Kuler - https://color.adobe.com/ Select Your Primary color and there are various ways in which the secondary colors can be found out i.e. Analogous colors, complimentary colors, triads etc.