How should be shosen to choose a ripe, sweet watermelon?

Already Very soon watermelons will appear on the counters of shops . However you must follow some rules that to buy a good watermelon.  Any ripe watermelon makes an echoing sound if you have lightly tapped on it him.  Also a ripe watermelon is covered with a thick, shiny rind shining ,shell . You must carefully examine the watermelon if there is to look for a rotten place with a dot in the center of him it. It is better do not not to buy such watermelon. You should must not bought buy the watermelon already cut into on the counter and adsorbed which has absorbed all the chemical substances from that the air.

Good job.  At the end of a sentence, put a period directly after the word, then put a space before the next sentence.   The skin of a watermelon is usually referred to as the rind.  A watermelon is an object so use "it" to refer to it (not "him" or "her").