Lust is simply when you feel attracted to someone on a sexual level. Infatuation is when you idealize the other person to the extent that they take over your whole world.
Love happens when there is action. The feeling must be mutual. Our personal definitions of love are constructed out of feelings and thoughts that we learned in childhood. How we experience and express love is the result of our interactions with parents, family and friends. 
The following emotions are often experienced when an individual feels love for someone else: happiness, exhilaration, excitement and joy. It may also be accompanied by negative feelings such as nervousness, insecurity and anxiety. Some people even feel butterflies in their stomach, stutter in front of the other person or don't feel very hungry when the potential mate is around. 
The shadow side of love is that it can also bring up feelings of anger, frustration, jealousy and separation anxiety (which means that you can't stand being without the individual.) These feelings however also often accompany infatuation. If you feel you must own or control another person, it is not love. True love gives the other individual his or her soul freedom. 
In a strong relationships the good will outweigh the bad aspects most of the time. However, realize you are a teen, and often your relationships will develop negative aspects so that you can learn from them. This is part of your journey towards finding a true soul mate in life.
There is a difference between loving someone and being in love with him or her. For instance, one does not love an object of desire quite the same way that they love their Mom or Dad. And they certainly don't love another person the same way a fat kid loves pizza or Ryan Seacrest loves hair gel. 
Being "in love with someone" is unique to every different soul on earth. Usually being in love means that you desire to be physically intimate with the person. Usually you experience elation or joy when you are with them and feel that you are somehow divinely connected. Sometimes it feels like fate or like you already have known the person in another life (that is if you believe in reincarnation!) 
However, be careful about feeling entitled to being with a person as some stalkers and obsessed types have thought they were in love while they were pursuing their victims. Feeling too divinely connected or being fated to be with someone is not a healthy sign in you or others. It is important to remember that if another individual does not share the same feelings as you do that they are allowed to say "no" to a relationship. They are also allowed to change their mind during the course of relationship and leave it without penalty from you.
In order to build a strong, loving relationship you need to choose a partner who treats you as an equal. The best loving relationships are built with people you can talk to and laugh with ease. Love is not a thing that feels "forced" nor should you have to pretend to be somebody else to get the approval of someone you fancy.
When assessing a relationship and trying to determine whether or not it is love you should also be very honest with yourself and the other person from the very beginning.