How can I figure out what 300 pounds is in American dollars?

By “pounds” I assume GBP (Great British Pounds), so…
Simple search on any of the major search engines available (my favorite is BING):

300 GBP to USD

You will also get a table were you can change the values of either GBP or USD and it will instantly convert without the need to hit enter (do another search).

If you are looking to convert other currency, simply add the forex designation for that currency or, if you don’t know it, search it or just write it all out. For example, you don’t need to write “EGP to USD” if you don’t know EGP. You can write “Egyptian pounds to USD” which is the same thing.