Read as much good, entertaining, engrossing, addictive fiction as you can.
For example if you like sci-fi, read The Expanse series. It’s 9 books. Thousands and thousands of pages, and it’s all entertaining.
Doing this will make you better at composing sentences of all types, short, long, simple, complex, future tense, past tense, complex construction, you’ll encounter hundreds of examples of each when you read a novel.