Here are some things you can do as a 13 year old:
Tutoring- Are you good at a certain subject? You can offer to tutor kids for money. Ask to talk to their parents and see what they say. You can say, "Hey, I am really good at math (or whatever subject the person needs help in) and I would like to tutor your child. I am charging (amount) of money. Is that something we can do? If they say its too much, you can make the price lower. If you need help with how much you'll charge, ask your parents.
Babysitting- you can babysit your neighbor's kids or your friends little siblings. Ask your parents if you need some help on how much to charge.
Walking or watching peoples pets- People need to go out places and need someone to watch their pets. You will have to feed the pet and maybe give them baths. Give the pet a walk and play with them.
Washing cars- Ask your neighbors if they want you to wash their car for a certain amount of money. Ask your parents how much to charge for this. You will need a bucket, sponges, water, soap. Wear some old t-shirt and something you don't mind getting messed up because you'll be cleaning a car. You might need a little vacuum too.
Lemonade stands- Look up on Youtube how to make lemonade and buy supplies at the store. Have your parents help you make it if you need help. Get a table, a box for collecting money, cups, napkins, a table cloth, and a sign that says what you're selling
If you're good at making things like bracelets or necklaces you can sell them online on Etsy.