What is the best gear to be in when you are going up hills?

Me and my bf bought mountain bikes from Walmart. (Mongoose) I used to be in a biking club about 11 years ago but that was on a road bike, and my skills apparently have declined greatly. I couldn't make it up a faiir sized hill at all and I want to know if I should get a better bike and until then, what gear should I be in to make it up the hill. I feel discouraged right now.