I'm sorry it took so long to get back to you!
Where you start varies a lot. I know about the US, and almost nothing about other countries, so anything I tell you is US-specific.
If you're employed and you have medical insurance through your employer, you should get a copy of the information on your coverage. It should have a list of psychiatrists you can work with. If it doesn't, and/or you don't feel comfortable asking your HR person or supervisor about it, you should see your General Practitioner, tell them what is going on, and ask for a referral to a psychiatrist. I would then work with the psychiatrist to figure out how to go forward: whether medication, therapy, or a combination of the two is the best approach.
I strongly recommend having your treatment handled by a psychiatrist and not a GP, because they are usually much better prepared to handle mental health issues.
If you don't have medical coverage, or really crappy medical coverage, I would contact a local org dedicated to helping folks with mental health issues. I have had extremely good experiences with my local chapter of Mental Health America. They don't have chapters everywhere in the US, though, so you may need to do research for your area. You can always call MHA, or you can also check out the resources listed on the Ask For Help web site.
That's my experience, which is again very US-specific. I hope this helps! If you need more assistance, just ask.