If there is one single skill that I can advise you to master that will greatly improve your relationship with people, it's Listening. Almost everyone loves people who actually listen to what they have to say. It might appear that extraverted people are more friendly but this is not necessarily the case (some extraverted people are just rude and annoying). Even quiet people are considered friendly when they show deep interest in the things you say. It is the people who know how to communicate that are considered friendly, and communication is a two-way thing. It's like a dance between two people where there is an equal amount of giving and taking. 'Giving' is communication is all about being generous with your listening ear - without forming judgment before the person has finished talking, without tuning out while the person is talking, or interrupting the person while he's still talking. To actual show an interest in other people (by honestly listening to what they have to say and responding) will make you very liked by many people. (And remember, it is by the way you respond that a person knows how well you have been listening). Listening also involves taking the time to carefully read a person's message or email before responding. Being quick to respond before you've actually digested and understood what was written amounts to NOT LISTENING.
All this might sound easy to do, but you will learn that listening is actually a very difficult thing to do for most people. This is because it requires an ego that is not selfish, arrogant, or narcissistic. Listening requires a person who respects other people and sees them as human beings of equal value. It is really difficult to listen to someone who you don't respect, or to pay attention when you feel you have more important things to share than what the other person is saying. Listening is really about seeing others as equals who have valuable things to share that can increase your knowledge in some way (this is the right attitude to have). Even is you're not social and outgoing, having this kind of attitude will draw people to you. Because whenever you act like you know everything or like you're superior to others, listening to them becomes difficult. And the more you ignore, misread, or undervalue what others have to say, the more they will think of you as a very unpleasand and unfriendly person.
So, good luck in becoming from friendly.