Let's say you have a person named Bob as a contact. In Contacts, select Bob and drag that contact record to the Journal icon on the Outlook Bar. A journal form will pop up with a shortcut to the Contacts entry for Bob and a space for taking notes, as well as a timer to time the call. You can delete the shortcut to the Contacts entry if you like.
After you complete the call, click Save and Close. Go into your Bob contact, and under the Journal tab, you'll find the phone call you just made.
If you initiate a call from the Contacts record directly (without following the above procedure) and the name or FileAs field are set a certain way, the call may not be logged on the Contacts record. See the MSKB article Phone Call Not Logged on Contacts Journal tab.