If you're an overthinking type maybe it is better for you to clear off your head(blank mind).Otherwise,if you're an emotionally intense try to describe all your feeling(why you're acting like that/is it going to make you a better person/is it good for your circle?(try to think rationally,everyone is different from yourself and please don't be hard on yourself by taking things personally).+ps:Do count your Blessings.
It doesn’t really matter because the point here is to hold something in your minds eye. So doesn’t matter what – could be your coffee cup that If you know well. I reckon in mindfulness meditation you can visualize anything. Or nothing at all. Just observe anything that comes to mind. I also started metta meditation (loving kindness) recently & found out that it’s good to start by visualizing someone you love and respect (not in a romantic way) and wish them to be happy and see how you really feel it. Then when you progress with your practice you slowly move to every other being and yourself. But the easiest way to start with loving kindness meditation is to start with someone you feel it towards already.