Having trouble with my patronus charm?

OK so i have a wand and already learned a couple spells and used them (in Hogwarts) don't worry i'm not planning on being expelled any time soon . Anyway I've been having trouble with my patronus charm. I've been practicing because there is a rumor going around that Voldemort will be using Dementors! Now it's just rumors but I still want to be ready. Onto my question, when ever i cast *Expecto Patronum!!!!* I think of the first time i received my wand (Craigslist). Is this moment not powerful enough? I don't have to many great memories. Any answers appreciated