How to approach potential supervisors who have already expressed an interest in supervising me after submitting my grad application? I am interested in a grad program at a Canadian university and alike US grad schools, this department requires me to submit a formal application first to be reviewed by the Grad Committee, get accepted first by them AND also secure the supervisor before I could get an offer letter. 

I have got a response from the Graduate Coordinator that they would like to admit into their program and have provided me with the names of two professors interested in supervising me. I would like to work with one whose work interests me, but how do I approach him? Do I simply say something that goes like, "thank you for showing an interest in supervising me, please share your current research projects" or something like, "I have read some papers from your lab group, and I have been impressed by Tom's (Consider him as a superstar graduate student in his lab) work and I consider myself an ideal fit for your lab." I am SUPER nervous &amp; confused! Please input your thoughts about how would you handle it.

Also, should I reach out to another professor who is also interested in supervising me? If yes, how should I politely decline him?
I don't think the rules require that you decline the other professor formally. I probably wouldn't do that. Reaching out to both could get tricky as you will need to choose at some point anyway. 

For the introductory letter to the one you prefer, introduce yourself and say something about your own interests and background (very short). You don't need to praise the professor. Saying that you have read a bit of their work and if some particular part of it is interesting to you then say which. 

I assume that you will get an invitation to meet since he/she has already expressed an interest. You don't need to praise yourself either. The prof will have an idea that you fit the requirements. It isn't a selling job, but an introduction and statement of interest. 

It is too early to make decisions. Wait until you meet to jointly figure out how you would fit in. But that would come after acceptance.