My friend which are Japanese always says きたー when he did something finished, then I asked him the meaning behind it. He said that it was similar よっす！ that kind of expression that you already did something clearly. But again, even my friend are Japanese, and can speak english, sometimes I doubt with his explanation because he always explain it on his own english version (which I found there are many mistaken here and there for word-choice). So what is the actual mean of that word?
きたー would mostly be used, as far as i know, would be either when something is finished being made or something you ordered has arrived. For example you ordered a pizza delivery with your friend, when it arrives you could say "きた、きた！" or its here, its here!, "きたぞう！" which is more along the lines of "it has arrived", or if you're a pretty calm person who doesn't have that much of a reaction to things, just "きた。" would work as well. Hoped that helped!