I will interpret this question as asking which SIDE of the tennis court do you serve into first in tennis.
The two halves of the tennis court are called "ad" (short for advantage) and "deuce." Your ad court is diagonal to your opponent's ad court and your deuce court is diagonal to your opponent's deuce court.
You begin serving on your ad side diagonally into your opponent's ad side. If you are facing the net from the baseline, the ad side is the right and the deuce side is the left.
In simpler terms, you start on the right side and serve diagonally into the left side of your opponent's court.
This is a link to a tennis court diagram. Perhaps a visual will help better understand the answer to your question. It can be found on the related links.
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The person who serves the first point of the tie breaker will be the receiver in the first game of the next set. R
Your first serve should be from the deuce side (that's the right side if you're standing on the baseline and looking at the net).