Take it on/upon yourself to do something
Definition 1 of 1
Pro Tip 1/3Personal Initiative
Implies you're acting on your own judgement without waiting for others to lead or instruct. 
She took it upon herself to report the issue before anyone else did.
Pro Tip 2/3Leader Impression
Using this phrase can suggest you are confident and willing to lead an effort or project. 
He took it upon himself to streamline the process at work.
Pro Tip 3/3Possible Annoyance
Be aware it might annoy others if you take control without consulting them first. 
Tom took it upon himself to rearrange the office, which irked his coworkers.
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