Your community is the collection of people who work on or with your project. Helping your student become familiar with those people makes them a more effective contributor during GSoC and helps to make them feel part of your community, which encourages involvement after their GSoC project is completed.
As a mentor, you can really help to set the tone for your student's initial experiences by facilitating community. Encourage your student to introduce themselves on mailing lists and IRC, and invite comments on their proposal. If your project maintains biographical information on contributors, ask your student to read through this information, and spend some time talking with your student about the people with whom they may interact.
Once a student has a feeling for who's who in the community, they are more likely to communicate and seek advice from others. This increases their chances of getting issues resolved more quickly and effectively than if they relied on only their mentor for help.
Asking effective questions is a skill, not an innate talent. There are many resources for learning how to ask questions effectively, and Eric Raymond's classic "How to ask good questions" (http://catb.org/~esr/faqs/smart-questions.html) is a great place for students to start. Another useful resource is Simon Tatham's "How to Report Bugs Effectively" (http://www.chiark.greenend.org.uk/~sgtatham/bugs.html) which isn't explicitly about asking questions, but does cover aspects of effective communication about technical issues.
Before your student asks a question, it's often helpful for them to take a minute to struggle with it and attempt to find the answer on their own. That extra effort often helps your student to solve their own problem. If not, it sharpens the question they ultimately ask.
Discussion in open source communities can be very direct. People will often criticize the bad points of a patch or suggestion, but fail to praise what is good about it. Explain to the student that comments aren't meant to be critical of them as a person, but are aimed toward improving the patch or idea and the project in general. Making potential new contributors feel welcome in your community is also important outside of GSoC.
Pro Tip: When you announce to the community that you're taking part in GSoC, make a point of explicitly asking people to make the students feel welcome.