I begin researching in three weeks and I have no idea where to begin. What should I say in my email to my advisor?
For starters, I hope you took her advice over summer. presumably this would be your first research experience, or an early one at least. With no prior practice your lack of knowledge is understandable. Presumably, also, your advisor knows of your state of experience. 

Given all that, just be honest. Where should I start? What should I read? I suspect, strongly, that she is experienced in getting people started. Otherwise you wouldn't have been accepted in the first place. 

But better than an email, expecting a long reply, would be a request for a meeting, asking for guidance. Don't try to overstate your abilities, and don't think that your current lack is a flaw. Everyone needs to start somewhere and it is good to have some guidance. 

But since she has already given you advice (which you took, of course), you want to ask her for "next steps" (rather than where to start) and mentioning your lack of experience in this whole process. It isn't a time for embarrassment. It isn't warranted. 



The situations in which this might be contraindicated is where the advisor is extremely busy - too many students, for example. But if she has a lot of students, it might be possible for you to join/start a group of her advisees and see what others know of her expectations. Some professors have weekly seminars for their students if they are working on similar things. Those other students can help you ease in to the process as well.