Your sixth form will probably teach you the basics but if you wanna get a head start then maybe try buying a book from amazon about the language (this is one I found: https://www.amazon.co.uk/dp/1840787198/ref=cm_sw_r_cp_apa_i_WbDPCbA68307Z) or watch some YouTube videos about the basics.
You can download Visual Studio for free from Microsoft's website too if you have a pc. That will probably be the IDE your sixth form will use in lessons. But as someone who's in lower 6th at the moment your best bet is to focus on your GCSEs and if you wanna get a head start then maybe try doing that in your summer holidays when you have got a lot of free time. If you've got any questions about sixth form then just ask.
Follow tutorials like Brackeys
Take notes with a pen and paper, review them away from the computer. Make some written notes /sketches of how you think code might work. Then work it out on the computer, when it goes wrong google.