The first thing you need to do is isolate the new kitty in a room with litterbox, food and water. Rub down the new kitty and the existing kitty with a towel and allow them to smell each other's scent. After about a week, stack two babygates in the doorway so they can see and smell each other. Do that for a week or so. Don't let the new kitty out unsupervised.
Time is what they both need. Time to get used to each other. There is going to be hissing and growling. As long as there's no bloodshed, you're okay. Since they're both quite young, I don't think it'll take too long. Have a squirt bottle ready just in case.
Good luck and keep us posted on their progress.
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