Where would you go? But more importantly, what would you do? It's a difficult question, but I would go to the 1600's about the time Isaac Newton finished the principa mathmatica (just because it's too important in history to distract him from).
The first thing I'd do (damn, this is where history bites me in the ass - the industrial revolution hadn't happened yet) is invent the typewriter.
I'll just take a moment here to explain that my aim in going back to this era of history would be to try and kickstart clean energy and computer power - lets say in an alternate universe that wouldn't affect my own history. The problem I keep having, is that in order to invent certain things, I keep needing to invent other things in the first place. Hence, with the typewriter, which would have a huge impact, and give me some wealth to pursue other interests, would ideally require a factory, which I don't have...
So, let's say I invent a typewriter, but they just have to be assembled slightly slower. But I've set the basis for the standardisation of the English language and the printing press, speeding up the literisation of the masses for a good few years.
Next... well I know where quite a lot of gold is (roughly) along the west coast of America, but I don't want to go there I'm busy. And since I don't know the exact details, I want to leave the natives be, and I want England to colonise the US anyway, I'll keep that ace up my sleeve.
Now my two aims are wowing Newton with my superior knowledge, and kick starting computing early (we still need electricity at this point, that's going to be a kicker).
So now I need to prove to Newton that not only do I understand the basic principles of his work, but also that I know about atoms, can write out the periodic table, explain the basics of quantum mechanics and related knowledge, like heizenbergs uncertainty principle, explain astrophysics, photons, bosons and various types of quark. Phew. I'd have to hold secret lectures, while somone records it for posterity with a typewriter. It has to be secret in case someone blows themselves up or decides to use it for evil.
Now the greatest minds in history have a roadmap to go down which they'll unfortunately have to do the maths for, but at least they know where they're going. Obviously the kind of refinement needed for some experiments won't be along yet, but hopefully we can speed that up.
Firstly we need some electricity (maybe we will need that gold after all) and an infrastructure to carry it around the UK. Street lighting is good, but I should invent LEDs, and patent lightbulbs so we can move over to low power brightness ASAP. As for the power source - probably will need coal power to begin with unfortunately, but since we don't have cars yet, and I'm keeping that secret, at least our emmissions aren't too bad. show workings for solar energy and wind power, cross fingers.
Now we need to invent the computer, which I'm sure some scientists can help me with if I show them the principles, starting with bulbs, and moving on swiftly to transistors. But we also need TV, and there I need help to. I know the principles, but I don't know how to guide the photons through the vacuum to the phosphor. But assuming we can do that, we now have a display for our computers.
I think I've got just enough time to invent the radio before keeling over from exhaustion and dying. Oh no, wait, I need to invent the internet too. that will be quite hard. I should have laid ethernet with my electricity cables. I haven't even invented the phone O___O
That should hopefully kickstart clean energy and computing, to the point we have enough of both to save the world before it gets desperate, and hopefully not turn it into a steampunk victorian age, or something horrible.
Anyway, it's pretty hard to think about what needs to be in place before you start going off doing stuff.