The most important part of resume building is finding the kind of things that ignite a passion in you and contribute towards the kind of career that you want. Worst thing you can do is see it as a tick box exercise, that said it is valuable to try as many things as you can while you have the space and time to do so - and to uncover any hidden passions you might not know about. Getting involved in clubs and societies is the most obvious one as is any sports or team games. Employers generally want to see that you can work well and contribute in a team setting as very few jobs today work as an island. Organizational skills are also desirable along with time management - if you can juggle volunteer activities with school work and keep your grades up this demonstrates that. If you have a clearer idea of what type of career or sector you want to work in its never too early to think about potential internship opportunities, or where these are not available have a look at the type of skills these jobs look for and see what activities you can participate in that show you capable.
Probably top of the list for resume building, but also largely invisible, is the skill of networking. Never be afraid of speaking to as many people as possible and showing an interest in what they are doing, how you can participate or solve their problems. This includes your group of friends as well as community leaders, employers etc. Increasingly the best jobs are ones which are not necessarily advertised but handed out through existing networks where you have proven your ability or shown an interest.
Good luck with your studies and resume building but most of all enjoy this time of your life.