In the first year you turn 70 ½ you must take a required minimum distribution from your 401k or traditional IRA. The IRS page that explains this in detail can be found here http://www.irs.gov/Retirement-Plans/Retirement-Plans-FAQs-regarding-Required-Minimum-Distributions 
When to start taking out
My father-in-law turned 73 this year and has not yet taken his RMD this year from his account. I wanted to take this time to research when and how much he needs to take out so I can be better help to him in advising him about his finances.
Normally you can take your RMD anytime of the year, but you must take it by December 31st. The one exception is the year in which you turn 70 1/2, for that year you can delay taking it until April 1st of the next year.
As an example my mother-in-law turns 70 ½ in 2014. She can take her RMD anytime between 1/1/2014 - 4/1/2015. The next RMD for her must be taking between 1/1/2015 and 12/31/2015.
What happens if you don't take it
To see what happens if you do not take your RMD you can view the IRS FAQ on this question http://www.irs.gov/Retirement-Plans/Retirement-Plans-FAQs-regarding-Required-Minimum-Distributions#8 
The simple answer is the amount you should have taken out gets taxes at a 50% rate. So if your RMD is $1,000 and you are in a 15% tax rate you would normally pay $150 in taxes on the distribution. But if you do not take the RMD by the yearly deadline you will get hit with a $500 tax (50%) vs the $150 (15%.... assuming you are in the 15% tax bracket)
Calculate the amount you need to take out
There are several RMD calculators out there here are a few I found. It’s a lot simpler to use these calculators rather than using the IRS forms to figure out what your RMD. I found all of these calculators to be accurate.
What numbers do you put in?
My father-in-law needs to get his 401k balance on 12/31/2012 to calculate what his RMD is for 2013. Always take the account balance on 12/31 of the prior year to calculate your RMD.
Using the calculator if I put in $100,000 as his balance on 12/31/2012 (which it is not) and his age this year, 73, and click calculate I get a distribution percentage of 4.220%. This number is based on his life expectancy. Giving a total RMD of $4,048.58
Keep in mind the number you put in there needs to be the total of all your 401k/traditional IRA accounts. So if you have $50,000 in a 401K and $75,000
The simple way
I find it very interesting to do calculation and understand the process, but there may be an easier way to get your RMD figured out. I just called my father-in-laws Traditional IRA asset manager (in his case J.P. Morgan) and because it is such a simple equation they can do it right over the phone with him, selling the shares in the mutual funds he has and sending him a check. (which is what he did)
I am still glad for learning this. And JP Morgan is sending him a check from his account that is the same amount the online RMD calculator.
I hope you found this article useful. J
 Retirement Plans FAQ regaring RMD
 What happens if a person does not take an RMD