Installing a timer forutside light. Box only has white, black, green. I know black to black white to white; but timer also has a red wire; I have no red wire in box. What do I connect the red wire from timer to? Right now I just capped it off, but on manual I cannot turn the light on

Well... I'll assume you are replacing a switch... the power to the switch would be connected to the black lead, all the whites would be tied together and the red wire would go to the light. The green wire gets connected where the other green wire(s) are, either with a wire nut or to a gounding screw in the box. Make sense?

installing a timer forutside light. box only has white, black, green. i know black to black white to white; but timer also has a red wire; i have no red wire in box. what do i connect the red wire from timer to? right now I just capped it off, but on manual i cannot turn the light on
This is a switch loop. There is no neutral in this box. If the instructions say to wire the white to a neutral you cannot use this timer in this box.

What is the brand and model?

Intermatic make an inwall timer that would work for this if you are not using flourescents.
http://www.intermatic.com/products/timers.aspx
Then click on in wall timers
This is nice:
http://www.intermatic.com/products/timers/in-wall%20timers/digital%207-day%20time%20switches.aspx