Calculate IC based on how bright you want the LED to be. (I'd say a hefty 10mA or so - it's a warning LED, you want it to be bright). Then calculate IB (it's IC divided by the transistor's DC current gain / hFE). Now, you want around 0.65V on the base (the usual threshold voltage), and you want the current through the divider to be around ten times the base current to avoid that affecting your calculations. Calculate the resistor to give you 0.65V, then make sure you'll have enough current. (If you won't, you will have to modify the design a bit, but I don't think it will be an issue.)
Of course, if you do find you don't have enough current, you could just choose a lower resistance thermistor, unless you've already bought yours. You also need to make sure you're not putting enough current through it to warm it up, of course.