His teacher couldn’t decipher his writing, so she gave him an F.

Your usage is incorrect. Use the word decide in that example.

So I give your quote of yourself speaking to your friend a big Hmm. I would say you have misused the word.

Then I’d advise you to use the word ‘decide,’ or the words ‘figure out,’ in regards to what TV show to watch.

For example, if your friends say they can’t decipher your handwriting, that means that your handwriting is hard to read. If someone on Yahoo Answers says they can’t decipher a question, that means that the question is written so that it’s hard to understand.

In the context of your conversation about which TV show to watch, I think the word you were looking for was “decide.” Hope that helps!