Some people like to do sports. We can group them into easy sports and hard sports. Easy sports are usually indoor sports. In summer, people usually play indoor sports such as skate, ping-pong, and badminton. Hard sports are usually outdoor sports. We can play them only when it is not too hot and not too cold. They are waterskiing, swimming, hiking, and running. We have to be patient to do these sports. However, these days people don't like to get sweat. It is also because of the hotness of summer and the coldness of winter. In summer, people usually do water sports in beach. Most of them swim or do waterskiing. They like to take a boat and cross the sea. In winter, we like to do something warm. But kids like to throw snowballs. Adults like to have indoor sports though it is hot, because it is good to let the sweat come out from their body.

Julie: In summer people usually play indoor sports, such as skate,
ping-pong, badminton.
Correct: In summer, people usually play indoor sports such as skate, ping-pong, and badminton.
Explanation: The conjunction and  is used to join two words or two parts of a sentence.

Julie: They are waterskiing, swimming, hiking, running.
Correct: They are waterskiing, swimming, hiking, and running.
Explanation: The conjunction and  is used to join two words or two parts of a sentence.

Julie: However, these days, people doesn't like to get sweat.
Correct: However, these days people don't like to get sweat.
Explanation: People is plural, hence use a plural verb don't.

Julie: And it is also because of the hotness of summer and the coldness of the winter.
Correct: It is also because of the hotness of summer and the coldness of winter.
Explanation: Do not put article the before any season.

Julie, great job! You just have to widen your knowledge about grammar and word usage to make better sentences.:)