I start working wid photoshop but when i save my file as .jpg it is ver heavy file (around 200 to 400 kb) why is that ??????
i have seen many .jpg photoshop files around 15 to 20 kb

what should i do ????

File >> Save for Web

File >> Save for Web

Yes, save for web. But also, you have to changed the optimized settings when you go to the save for web window.

But if you want good quality, just leave it as it is.