For the first question, I would base my search on things like the extensiveness of the results, authentication, pricing, usability, etc. If you don't have any specific criteria right away, try making a few calls and playing with it to see if it will be what you needed. Some APIs I'll find its missing this one thing that makes another api a better choice. This is something you'll get better at with experience.
And no youre not expected to know all the tools, there are about an infinite out there. Generally youre expected to be familiar with some of the tools of the job but knowing more can't hurt either. Take things one thing at a time and make some simple project with what you've learned. That's how I do it anyways, but everyone is different.