Q:

Where does action based on thought originate within the human brain? 

A rather basic question, but I don't see answer in Google, so here it goes: Humans have thoughts, sometimes these thoughts carry with them motivations to act, urges, cravings, etc.  All these can make a person start moving to accomplish something.  

For example: "I'm rather bored, I think I will go get a snack".  What
  happens the moment I cross the threshold from thinking to actually
  getting up to get a snack? 

What happens the moment the person starts to act based on a thought?  Where in the human brain does a thought trigger action? 
Update: In response to an answer suggesting that in many cases action comes first and rationalization for action comes second:
The kind of action that I'm talking about comes after a longer period deliberation, where the current state is being weighted against the benefit of the action to take. 
For example in the situations below, a person can deliberate for minutes:

A person is in bed and is very close to sleep, being comfortable and relaxed.  Loud noises from the outside disturb the person's attempt to fall asleep.  In this case the person can experience conflict between staying in bed (comfortable but disturbed) or going to find earplugs / shut the windows (uncomfortable, but potentially lessening the disturbance)
A person is trying to meditate, thus relaxing the body and unfocusing the mind.  An itch on the foot is disturbing the meditation.  Such person can either continue the current state while ignoring the discomfort or scratch the itch and potentially lose the meditative state. 

In the examples above, two conflicting courses of action are evaluated by the brain.  Due to the ongoing discomfort, it is possible to stay in this state of "almost making a decision" for quite some time.   At a certain point the scale can tip into taking action to resolve the discomfort.  What is it that causes the person to take action as opposed to inaction? 

A:

There are many areas of the brain that are associated with planning complex behavior.  This is because planning and executing are mediated by the brain's capacity for executive functioning, and EF is further associated with many areas of the brain -- in this case, the the frontal lobes, the prefrontal cortex, the caudate nucleus, and the putamen all seem to play a role. 
What typically happens is that a thought is formed ('I'm hungry, I want a cookie', etc.),  and then potential actions are envisioned and evaluated for their predicted effect, in line with the common coding theory of the relationship between perception and action.  From a functional standpoint, the caudate nucleus selects the appropriate action based on an evaluation of the outcome of the action. [ 1] [2]
The more complex the task, the more cognitive strategies you may need to use, and these are mediated by your executive functioning ability.  Cognitive shifting or task switching, information updating and monitoring ("updating"), and inhibition (or the 'tuning out' of irrelevant stimuli) all play a role in planning and executing complex tasks, or sequences of behavior. [ 3] For example, the ability to act in momentarily unpleasant yet beneficial ways (i.e. going to the gym) requires cognitive inhibition of more immediately gratifying, yet irrelevant stimuli (i.e. sitting on the couch, eating pleasurable food).  For tasks of this nature, the frontal lobe plays a greater role. 
Certain neurotransmitters also play a role in your ability to stay focused and dedicated to a task.  Dopamine in particular is responsible for motivation (including the motivation to act), and norepinephrine is responsible for vigilance of attention, thus increasing the likelihood that an action will be performed and completed successfully.  With a reduction in these neurotransmitters, you may demonstrate attentional shifts.  Long-term, uncontrollable shifts in attention are often representative of an underlying disorder, such as attention-deficit disorder. 

Sources used:
[1] Grahn, Jessica; John A. Parkinson; Adrian M. Owen (12 April 2009). " The role of the basal ganglia in learning and memory: Neuropsychological studies".  Behavioral Brain Research 199 (1): 53–60. 
[2] Oury Monchi, Michael Petrides, Antonio P. Strafella, Keith J. Worsley, Julien Doyon (Feb. 2006).  Functional role of the basal ganglia in the planning and execution of actions.  Ann Neurol.  59(2): 257–264. 
[3]  Miyake A1, Friedman NP, Emerson MJ, Witzki AH, Howerter A, Wager TD. ( Aug. 2000) The unity and diversity of executive functions and their contributions to complex "Frontal Lobe" tasks: a latent variable analysis.  Cogn Psychol.  41(1):49-100.