Somebody who is a "ya d00d" is basically a huge douche. These are the guys who's list of friends consists of assholes, people who like to get drunk or high, posers, douchebags, or guidos. A ya d00d is essentially a less conspicuous guido, as they are usually dressed as the stereotypical teenage douche. Sideways/backwards cap, ironically childish backpacks, etc.
A good way to tell who a ya d00d might be is during online communication. When asked a yes or no question, if the answer is yes, they will most likely reply "ya d00d". In certain cases, a ya d00d might actually replace all the o's in a sentence with a zero instead. Ya d00ds often consume a lot of alcohol, and will talk about the act more than they actually do it. A standard ya d00d will not shut up about how often he gets drunk, or how often he smokes marijuana.
If at all possible, try to avoid any ya d00d you may see. They are a hazard to the general public, a danger to your mentality, and overall are just fucking awful. The term can be heard in a line from the song "Those Kids Suck" by Big D and the Kids Table, where he says, "I'd rather be called nuts than to be a ya d00d."
Person 1: Why is Matt even friends with that douche?
Person 2: Well, Matt likes getting wasted. That kid's a ya d00d, so he has a lot of alcohol at his parties.
Person 3: Yeah, to be honest, if he didn't have that much beer, he probably wouldn't have many friends. Nobody likes ya d00ds.
ya d00d: w0w d00d ur n0t c00l mayne
Rational person: Shut the fuck up.
Person A: Why does he have such a hot girlfriend? He's a total ya d00d.
Person B: I guess girls are into complete assholes.