You know, it's really a matter of what you want. If you want to take the risk of trusting someone, then go for it. You could get a great friendship, or lover, or anything really. Or maybe they'll stab you in the back. You never know, really. If you seclude yourself though, you'll always have that hint of doubt. My suggestion: Observe people's behaviors while you first meet them, and then decide if they seem trustworthy or not. Good luck!
Neither. As you get older learn to trust your instincts, only. They become pretty sharp over time. In the past I always trusted people until they gave me reason not to trust them. Even when my gut said something was not right. Then I'd end up hurt. No matter how impolite it seems, be patient about forming friendships & saying or doing to much, too quickly.