To expand on @MDMoore313's answer, this does depend on how immediately "now" needs to be.
You can definitely and very easily set a package, or script, up to run as soon as any machine in a group, or collection, next calls back to the server to check for new jobs, but by default there is no way to send out a broadcast to all machines to run "NOW!".
After you've created your Package/Application in SCCM with your program or script in it, you create a Deployment that has the instructions in it with who to run this on, and when (amongst other settings), on the Scheduling page of the Deploy Software wizard you'll find the Assignment Schedule, where instead of setting a date/time you can pick "As soon as possible" which means that the assigned machines will start downloading and running this job as soon as they know about it:
The timing of this is dependant on your Client Policy Settings, these dictate how often your machines poll an SCCM Management Point to check for updated policy and new jobs. Even if you set a deployment to "As Soon As Possible" it won't be run by a machine until after it next polls the SCCM servers for updated policies.
In SCCM 2012 you can find the current "policy polling interval" by going to Administration -> Client Settings, selecting a relevant policy and then going to the Client Policy tab. (note if you're going to change any of these settings it is recommended to leave the Default Settings as they are and create a new Custom Device Setting to over-ride the default set).
However SP1 for SCCM 2012 also has a new Client Notifications feature that is "a fast channel that a Configuration Manager administrator can use to notify clients to initiate time-sensitive tasks as soon as possible." Using this, after setting up a program/app and deployment (set to "As Soon As Possible" you can then right-click the Collection of PCs (or multi-select machines from within a collection) and select "Client Notification" "Download Computer Policy" to notify the machines that they should poll for new jobs immediately (you also get a pop-up notification showing how many of those machines are currently online and responding to notifications).
More info here: Fast Channel for System Management - Client Notification in System Center 2012 Configuration Manager SP1