Add up what each item really costs
Start with what you paid for the item or its materials. Then add packaging, the share of transport to the market or supplier, data for posting, and anything you pay others to make or pack it. That total is your cost, and your price must be above it.
Choose your profit on purpose
Decide how much you want to make on each sale, as an amount or a percentage, and add it to your cost. Look at what similar sellers charge, but don't race to be the cheapest: buyers also pay for quality, fast replies and reliable delivery.
Price each option on its own
A bigger size, a premium colour or more storage usually costs you more. Give each variant its own price so you don't lose money on the expensive ones. Each variant can also have its own stock.
Plan for discounts and delivery
If you'll run sales, leave room in your price for the discount. Charge delivery separately by area so a far delivery doesn't eat your profit, or offer free delivery only above an amount that covers it.