I'm friendly! It takes a while to build up the confidence, though, and that's the part that's not necessarily natural for INFJs.
My pointers:
Remember, if you say "good morning" to strangers with a smile when they pass, they may give you weird looks. However, that's mostly because it's not as common as it should be. It's uncomfortable at times, but few people dislike others because they greeted them
Don't let that get you down!
If you smile and ask someone how they are doing, generally they will react positively unless they are having a bad day or are just generally grumpy. If you give a person the opening to talk about themselves and how they feel, they will either do so or they won't. Take the cue as to their reaction on whether to continue or not. But remember, if they react negatively, it's rarely because of you -- it's usually because they're having a bad day. Just stay positive and go about your business.
Weather, other person's day, common experiences (what is the setting?), or other common tidbits are good conversation starters. One of the hardest parts is getting going.
Eye contact doesn't necessarily have to be constant -- in fact, sometimes you can avoid it altogether. Just try not to look down or lower your eyes (that shows submissiveness, shyness, uneasiness, nervousness -- it kind of closes you up. People react like mirrors; if you're uneasy, they may very well become uneasy too). I usually look at their mouth/lips, forehead, etc. when I don't really feel comfortable with direct eye contact.
Watch your posture. Body language says a lot. Back straight, head high, shoulders perky, and a smile really says "I'm confident and inviting" even if you're not really. Learning how to control your own body language can help you control your situations, and, sometimes, even how you yourself react to a situation.