I haven't said much about exactly what it is you are supposed to think about people when you do loving kindness. The basic reason for this is that the details don't especially matter. You should feel free to experiment a bit. Just keep the basic idea firmly in mind: the goal is to think universal kind of thoughts about specific people. What I mean by this is what you wish for someone should be something you would wish for anyone you care about. So don't wish for Billy to do well on his organic chemistry test, but it would be fine to wish for Billy to experience unconditional love.
That being said, I will add two more specifics. First, it's ok (almost unavoidable) to repeat. Don't think you have to wish 50 unique loving things about somene for 5 minutes. If you want to just think about 4 things and repeat them for five minutes, that is perfectly fine. Second, here is a strategy I myself find helpful and you are free to use it yourself, modify it or ignore it altogether as you wish:
I find it helpful to link wishing that the person does not have bad things with wishing that they do have good things. Here is what I mean:
Breathe in: may Billy not suffer from illness or pain
Breathe out: May he instead experience health in every aspect of his mind and body
Breathe in: May Billy not experience impatience with daily irritations
Breathe out: May he instead develop a deep sense of patience
Breathe in: May Billy not experience feelings of low self esteem
Breath out: May he instead know that he is a human being of infinite value
Breathe in: May Billy not succumb to feelings of anger
Breathe out: May he replace his anger with love towards others
Breathe in: May Billy not yearn for what he does not have
Breathe out: May he instead feel contentment with his life
Breathe in: May Billy not be pulled in many directions by competing desires
Breathe out: May he instead learn what is truly important in life so his path is clear
Breathe in: May Billy not act impulsively
Breathe out: May he instead learn to always be mindful and govern his actions with reason
Right, so let's do another 20 minutes of loving kindness meditation - like it or not...:)