Hi there,
Congrats on your upcoming marriage
Firstly, I would suggest that you start gathering all evidences from now. Phone records, chat records, any gifts you have exchanged (reciepts of those), if you have had an engagement, then pictures from the engagement, if you guys have gone out together, how the marriage came about, evidences of that.
Take lots of pictures at the wedding, showing the guests (if its a typical indian wedding, you get loads of pictures by default ;D), then your honeymoon pictures, after-wedding pictures etc. Remember that when you finally assemble your application together, your relationship should read like a story, explaining everything from how you guys first met to the current times. It is advisable to make it in a chronological and simple manner, making it easy for the VO to go through.
Also find out the times taken to get the Police Certificates from your respective area. Depending on where you live, a Police Clearance Certificate (PCC) can take anywhere between 10 days to 3 months. So it would be a nice idea to get that started on right now. However, remember that a PCC should not be older than 3 months when your application reaches Mississauga. So say for eg, you apply now for a PCC and get it next month, but your marriage is not until 2 months later, and it will take you more time to get your application in, the PCC would have expired by then. So act accordingly and find out how much time a PCC takes in the area where your spouse lives. This should help you http://www.cic.gc.ca/english/information/security/police-cert/asia-pacific/india.asp
As for medicals, they are valid for 1 year from the dates they are done. Medicals are usually the last thing you do, right before you submit your application. Remember that once a visa is issued, it is valid till the time the medicals expire or the passport. Here's a list of the Designated Medical Practitioners (DMP) you need to contact depending on your spouse area: http://www.cic.gc.ca/dmp-md/medical.aspx
Hope this helps.
Shamsia