What is the easiest to upgrade my chainrings from a double to a triple? I have a 2007 Specialized Tarmac Pro Double and I need some better gearing. I am doing more hill climbs and I feel that I need at least two lower gears than I already have. What would be the most efficient way to upgrade my bike. Would it be easy to get a triple chainring? Will I have to get a new derailleur? What else would I need to change?
You have a couple different options. I think the cheapest would be to switch to a compact crankset which would change the front chainrings from 52-39 to 50-34.  I'm pretty sure you could do this while still using the same shifter. Changing the front shifter is required if you want to go from a double to a triple, as is changing the front derailleur.  If you switch to a compact I'm pretty sure you can keep the same derailleur and shifter.

Another thing you'll want to look out for is the rear derailleur.  You currently have a short cage derailleur, and you may need a medium or long cage derailleur when you go with a compact crankset.  

I don't think a triple should really be necessary on a 10 speed system.  I have triple, but I have an 8 speed cassette. I find in that case the triple is nice because you can still get lots of gear range while still keeping the gears close together, However, on a 10 or 11 speed cassette, this isn't really necessary.  I think that switch to a compact crankset is a much better option.

Also, according to This post, the maximum cog size of a short cage Shimano short cage is 27 and the total drivetrain capacity is 29.  To use a larger cassette, you would have to switch out the rear derailleur. If you used a compact crankset, you should be probably also replace your rear derailler for a medium cage, as your total capacity would be (50-34) + (27-12) = 31.  However, if you changed the cassette to a 12-25, you could get away with not replacing the rear derailleur and still getting a little bit lower gearing, although possibly not enough for your needs.