Start with the basic stuff, rule them out first.
1) Battery - when you say it is "good", what do you mean? The only way to know for sure that the battery is in good enough condition to power the starter is to have it tested while a load is put on it (simulating an actual start attempt). You can buy testing equipment to do this, but it's not normally something your average shade tree mechanic will have. Best way to test it is therefore to take it to a local auto parts store and have them do it.
NOTE: the battery has to be fully charged for them to test it. So if you take to them without a full charge, they will tell you they cannot test it yet, but to come back later after it's charged for a while and they can test. So save yourself some trouble and make sure it is fully charged before you take it in anywhere to have it tested.
2) Starter - the starter will also need to be tested by applying amperage to it to simulate a start event. The only way this can be done safely and reliably (meaning you can trust the results of the test) is to remove it from the engine and take it to an auto parts place or repair shop and have them put it on a testing machine meant for that sort of thing.
NOTE: back in the old days we used to do this testing ourselves by pulling the starter and battery off the car and connecting the +/- from the battery to their respective posts on the starter. I cannot in good conscience recommend that you try this though. It might be easier than taking it into have it tested somewhere, but it's certainly not safer. Also, the results of such tests can never be fully trusted due to the amount of Jerry-rigging used to perform the test, right?
3) Wiring - Every connection, bend, joint, etc. in the wiring between the battery and the ignition and the starter and the ignition, etc. is a possible failure point that could prevent starters from engaging. So ideally you would want to check everything here as well, but in reality, if there are not other electrical problems, and you can verify the voltages are what they need to be at the critical points in the picture, the wiring can be ruled out as well.
If all three of those turn out to be good, then you will have to move onto the hard stuff. But that will involve lots of back and forth with the PCM, which is possible for us shade tree mechanics nowadays, but it is much easier and quicker for a pro to do it using the correct equipment. So if you rule out the three items above, I would suggest giving serious thought to just having it towed to an automotive repair shop and letting them take over. You might spend more money going that route, but then again, you might save money by not having to repair the things you would have messed up by going at it yourself.
Good luck, and don't forget to update this thread when you figure out what the problem was!
Gert