If you are a native English speaker with reasonably good writing skills, one of the easiest ways to earn money straight away is by signing up for sites such as UpWork. Create a good profile with samples of your work and start applying for jobs. Initially, you may need to start with jobs that are quite poorly paid to build up a good rating, but then you should be able to earn reasonable money for your time.
My 10-year-old son brought home a book from our park’s free library box. It was a biology textbook – teachers edition. He said it looked interesting and hey, it was free (having no idea you could sell it). I scanned it in my Amazon seller app and realized it was worth around $150. He was so excited. We listed it for sale for $130 and it sold! Going to tell him, he just made $130!