to start, think about how YOU would add the first 100 numbers and store results. don't think in
Java, think in English, Spanish, Korean, or whatever language you are most comfortable with.
be specific. don't just say "i'd add them up and write down the result."
HOW would you add them up. how would you make sure you got all of them? where would you store the result (or any intermediary result you may have along the way)?
once you answer these, writing the Java becomes much easier.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors