1999 Toyota Camry Starting on its own

For three days in a row my car would not start on the first try in the morning, but would finally turn over after 3 or 4 times of trying. Then for the rest of the day it would not give me trouble starting again. The last time I was able to start it, I went back inside my house for 2-3 minutes and when I came back out it had turned off and would not do anything (no lights would turn on or anything). We tried to replace the battery but when both sides were attached, sparks started flying and it sounded like the engine was trying to turn over. Any ideas? The keys were not in the ignition when attaching the battery and my car will not let me even try to turn it on anymore.