From http://docs.python-requests.org/en/latest/user/quickstart/
url = 'https://api.github.com/some/endpoint'
payload = {'some': 'data'}
headers = {'content-type': 'application/json'}
r = requests.post(url, data=json.dumps(payload), headers=headers)
You just need to create a dict with your headers (key: value pairs where the key is the name of the header and the value is, well, the value of the pair) and pass that dict to the headers parameter on the .get or .post method.
So more specific to your question:
headers = {'foobar': 'raboof'}
requests.get('http://himom.com', headers=headers)