how i start securing my web? i mean all the step

recently, just finished information system. using php, html, javascript, and mysql with native framework. and there's someone told me that i have to make the web secure because native is not secure enough. but i dont know where i have to start it, i'm totally new in this "security website" things. i'm looking on youtube but they only demonstrate it in simple php page and when i try, i dont know whats going on and if i try to do like in the video(like sql injection) it failed please give me some tips and maybe good references how and where i have to start it thank you