PANTONE and color conversions
I have been given a standard color palette. The colors are defined in terms of
PANTONE #### X
PANTONE 1234 C.
Followed by their CMYK, RGB and HSB values.
Another page defines Tints and shades. For example, PANTONE 1234 C TINT = 50% .
How can I convert this particular tint to a given RGB?
R = 0
G = 101
B = 96
How do I adjust these values for 50% tint? Second, what is and how do I lookup a PANTONE?
To answer your second question first, Pantone is a color-matching system, like Trumatch or Toyo. It's just a standard so everybody can agree on what "kelly green" is.
In Photoshop, click on the Set Foreground Color box in the vertical toolbar. When the Color Picker comes up, click on Color Libraries. In the dropdown menu at the top of the box, you have a slew of Pantone libraries (which are just groups of swatches). So you have various sets of pastels, metallics, process, and solid colors, which shift slightly depending on whether you're printing on coated or uncoated paper.
In Illustrator, go under Window and select Swatches. In the Swatches palette, click on the Options icon (the three lines with the down arrow, not the two right arrows) and scroll down to Open Swatch Library. From there select Color Books. (Yes, Illustrator is annoying this way.) The same list as in Photoshop should pop up.
To get the RGB values for the tint? Honestly, I'd cheat. In Photoshop, I'd select my Pantone color (for the sake of argument, Pantone 123, a sunny yellow). Make an new document 100 pixels square and fill it with the yellow. Under Layers, change the opacity to 50%. Flatten it. Eyedropper the result. I get 255/227/150.
You don't say what the end product is supposed to be, so I can only give with limited advice.
To find specific Pantone colors as RGB swatches, use this page on the Pantone website. Since you have the RGB values already, you could also just type them into the color picker of the gimp (or equivalent).
To get tints, follow Lauren's suggestion, starting with the RGB values from your specification sheet. Use the equivalent procedure in whatever RGB image editing software you have available.
Your most accurate and certain course would be to purchase the Pantone Solids swatch book, but if everything you are producing will be seen only on a monitor or mobile device, accuracy is neither attainable (since you have no way to calibrate the viewer's screen) nor much of an issue.