I'm a student but I also do some day trading with stocks. When I report the gains I made, do I report (amount I paid) - (amount it sold at) and then based on the result, the tax?

For example, I do trades but on average I'll make say 50-100, I only pay tax on that gain of 100 on the trade? right? Or do I have to pay tax on the entire 10,000 trade, even though i only gained 100 out of it. thanks.