Think hard and explore the options of different pathways you can take in graduate studies. Make the best out of grad school, be social and go to events to meet others in your program. Enjoy your time here, it goes by extremely fast.
Do your research: on the lab, on the faculty, on the department and on the city itself. There are so many different programs/departments and it's imperative you know what you're getting into. Speak to students that are currently in the lab or have recently graduated to get an idea of how things work. There are going to be some tough times in your program and you want to make sure you're surrounded by people that want to support you and help you get through, to be successful.
To any undergraduate student that wants to pursue graduate school, my best advice would be to study hard, celebrate your academic victories, and don't get caught up on your failures. Everybody has a story of a course they wish they did better in, or a midterm/exam that was an unexpected curveball, but as long as you learn from your past and keep pushing forward, things will work out for the better.
Smart or slow, doesn't really matter in the end, the point is everyone has their own time zone, all you have to have is your own balance of work, play, and all other stuff, the others don't really matter.
Make sure you need grad school for your future career before you do it. Reach out to other grad students while you're in undergrad; most people will be more than happy to share their experiences.
Try your best to reach out to faculty and make connections with people who share the same passions as you. Everyone at the University of Guelph is happy to share their expertise and provide guidance, and those connections are very valuable in expanding your knowledge and reaching your goals.
Pursue research that you truly believe has the potential to improve life. Knowing you are working toward something more than just a degree can help you find the drive to make it through the difficult or dry aspects of your time in graduate studies.
Your time in graduate studies will provide many opportunities, but don't lose touch with yourself. Take the time to do what you need to in order to maintain your sense of purpose and direction, then live out of that. Don't be afraid to try new things, but remember that no one can do everything. Be selective.
To future graduate students, it is important to keep a clean, legible, and logical lab notebook! Also remember that your brain is your best weapon to succeed in graduate school and it functions best when it is well-rested; only pull all-nighters if necessary. Learn to manage your time and try not to do multiple things at once; multi-tasking in the lab can lead to errors in experiments. Finally, enjoy your time and get all the food and freebies from conferences and other events!
Although I think most prospective grad students have been told about the importance of picking a study/project that interests you, I think it's also really important to take some time to see how you interact with your supervisor and lab. These will be the people you work the closest with for the next several years, and help provide support to you in some of your toughest moments.