Thanks JsT, I'm sure I'll be okay in the end.. it's just a bit scary thinking about all the things that could go wrong!
Franki is totally right. The most important thing is to give yourself lots of time. Did you notice that on journeyplanner.tfl.gov.uk you can check the timings for the day and time you're travelling? That can be really helpful.
There is a bus, if you'd rather, but it takes a long time. The 73 from Victoria runs to right outside King's Cross mainline.
Also, at any time, you can always find a member of TfL staff to help. If you urgently need assistance, there are call buttons on the platform, otherwise you can head to an exit to find someone.
Promise me you'll always remember: you are braver than you believe, stronger than you seem, and smarter than you think.