You have to get the key first, but programming your fob is easy.
Iirc, you turn the ignition on and off 8 times, leaving it on the 8th time, then press a lock or unlock button on your door, then press the button on your fob.
When I bought my car, I only had one key and fob, I bought another fob at a swap meet for 10$ and the lady told me how to program it. Took like 30 seconds in the parking lot to do it, and I think that's what I did. As for the key, luckily about a year later the guy that traded it to the lot I got it from found me and told me he still had the spare key (could have been a scary situation) but he came to my work and gave it to me one day.