Welcome to the newbies!
isabelD - if the writing is fantastic, the agent isn't going to care whether it's single, double or triple spaced. However, you should probably take a few moments to scour this site and the web to learn a little about the standards that way you will present yourself as a professional. Standards are pretty much double spaced, TNR12 (some prefer Courier New 12), one inch margins all the way around and a header with your name, book title and page number. Also, don't forget to include all of your contact information in your query. And, of course, follow any specific information found on the particular agent's website.
Best of luck to you and congratulations on starting your submission process.
Julie