Ah, another Oregonian! We Portland-area folk live in a virtual paradise for hiking--the Cascades, the Gorge, the Coast Range, the beach, and even a 7,000 acre mostly wilderness park (Forest Park) in the middle of the city!
I highly recommend this site:
PortlandHikers.org Their home page has links to the Portland-area hiking clubs and to other groups that regularly sponsor group hikes. Going with one or more of these groups is your best way to get started and gain experience. You don't have to be a member to go on the hikes. Some of the groups also schedule backpacks, which would be your best way to get started in that area as well.
There is a ton of info on places to hike in the PortlandHikers.org Field Guide.
Be sure to read the articles on this site's "TLB Home" page (see "site links" to your left)--they're listed in the left-hand column. The "Hiking Essentials" and the "Day Hiking Gear List" are particularly important for a beginning day hiker.
And welcome to this forum!