Find the closest bike shop to your place. Getting to know your neighborhood shop mechanics is always a good idea. But, Seven Corners Cycles on SE 21st just north of Powell is always a great choice. Couple of years ago, I called needing a tube and they left one outside for me since they were closing, and said, "just pay the next time you come in." That's service you can't beat.
I also really like Joe Bike on SE Hawthorne at about 41st. Always helpful and Joe's a good guy.
If you've got limited funds, CityBikes has open shop every other Sunday. They've got mechanics who will answer questions and help you get it done. It might be $5 suggested donation.
If you've got a bike and little to no cash, come out to Grace Bible Church on Sundays. Wrench Raiders is a nonprofit who fixes bikes for free. We can help you fix it, teach you as we fix it, or just fix it for you.