Well by default the Swatches palette should be open. On that palette should also be Color tab. If you don't see it then go Window/Swatches or Color to open the palette.
Also can go on the Tool Bar towards the bottum where you have your foreground/background colors.(default black and white) Double click on the foreground color(default black) to bring up the Color Picker window.
*make sure you're painting on the top transparent layer and not the image layer.
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Since your new, I'd like to add to my previouse directions
8. To save your your composition, go File/Save (always a good idea to save often while working incase of a computer or program crash)
9. To save a file for web presentation, go File/Save for web. Select file format you want. .jpeg is cool. It's a good compression which yields good quality. Turn the quality to 100% if desired. Save.