I live in Washington state and have been trying to figure out what the laws are on registering your handgun or just any firearm.
All I can really find out so far from searching online is that it is not illegal to own a non registered handgun, but that if you use it in self defense they will slap you with a fine for possessing a "illegal" gun.
So that makes so sense.
Also what are the laws for a gun with no serial number?

Thanks for reading, I just want to know what I'm getting into if I need to defend myself or my family

There is no compulsory gun registration in WA.  I have heard of a voluntary registration that some claim can help you get a stolen gun back.  I don't know how it works or where to go to do it.

What does "searching online" mean?  Did you search the WA RCWs?  If not, what website are you citing?  

Getting charged with possession of an illegal gun can only happen if the gun is illegal in the first place, and registration doesn't make one legal or illegal.

What gun?  Modern handgun?  80% gun that was finished?  Antique firearm that never had a SN to begin with?  

I would suggest reading Dave Workman's book, Washington State gun rights and responsibilities.

There is no de jure registration in WA. There is de facto registration of handguns when you buy one since the state's Pistol Transfer Application is also sent to the state and they keep it filed. However there is no law that says you have to "register" handguns that you acquired before I594 changed RCW 9.41, or that you were gifted from an immediate family member, or that you had purchased prior to moving to WA. There is nothing that says you can get slapped with having an illegal gun unless the gun is actually illegal. Like one with the serial number scratched off, an unregistered NFA firearm (maybe that's where the confusion starts?), or stolen.

Where did you read that fucktarded nonsense?
We can't help you out unless you help us out.
Link? State Agency? Police Department?

Where did you read that assfucked shitstained retarded nonsense?
Link?

My suggestion is for you to stay off the internet.

If it were ME and I was going to keep a firearm for SELF DEFENSE or use one for self defense I would BUY a modern LEGAL firearm that was manufactured by a reputable firearms manufacturing company and use LEGAL modern ammunition.
It would not be some crazy expensive firearm and it would not be some clapped out POS homemade firearm.

January 26, 2015

TO: Washington State Firearm Dealers

RE: Discontinue Use of the Pistol/Revolver Private Disposition/Transfer Form #652 004

On December 4, 2014, Initiative 594 “Background Check for Gun Sales and Transfers” took effect.
As a result, Department of Licensing’s (DOL) Firearm Program will discontinue use of the Private Disposition/Transfer Form, #652 004.

The discontinued form has been removed from the DOL’s website and we ask you to discontinue use of this form and to destroy any copies of the form you may have in your place of business.

The Pistol Transfer Application Form, #652 001 is the authorized form to report:
• A sale or transfer of a pistol from a Licensed Firearm Dealer, and
• A sale or transfer of a pistol between private parties through a licensed dealer.

Please refer to RCW 9.41 for additional information on Firearm License Dealer reporting requirements.

If you have further questions, please email us at [email protected] and you can call us at 360-664-6616.

Sincerely,
Karla Laughlin, Administrator
Public Protection Services Section
Firearms Unit

The form #652-004 was VOLUNTARY for PRIVATE PART PISTOL/REVOLVER transfers / dispositions.
No longer used.

This is what it looked like.
http://www-dev.dol.wa.gov/forms/652004.pdf