I was an undergrad researcher in a condensed matter group, as well as a nuclear physics group, and worked for a cumulative 3 and a half years. At the undergrad level, what the PI is looking for is enthusiasm. As long as you seem excited about the work (and have decent grades), you should be good. Make sure you read up on what the PI is doing, and what areas he or she is working on that you find particularly interesting. Try to come up with questions on what you find out, and don't be afraid that they are stupid questions, PI's are more than happy to explain their work to you. They will teach you what you need to know, but they can't make you enthusiastic.