Once you have taken the self-assessement test and if you have the required points, then by all means get your application for or do it online.
But as suggested, do finish your schooling first, but, you can save alot of time by starting to prepare for migrating to Canada, but getting all the documents required. That is what takes most time..making sure you get all documents together.
Make sure all copies are stamped by a notary public
Send in a original Birth certificate
When you do apply pay all the required immigration fees upfront ..one time payement..it will save you months and money. As they dont have to write to you and then you send to them, then they have to process etc. If you are not accepted they will refund your money back with the exception the application fees.
If you use a consultant..make sure the consultant or lawyer has a canadian liscence hanging on his wall, take his or her number and check up with immigration to see if they are on the list of qualified and registered consultants and lawyers to prevent you from being frauded.
If you do it on your own, make sure you triple check everything and if you have any children out of wedlock..DO WRITE IT..many think not important..but it is and you could be refused and accused of fraud. Always declare everything, don't miss anything. Take your time. Be a turtle.
U can do your prints, medical, police certificate and send it at the very first application, if they wish a medical again they will ask, if they dont this saves you alot of time of having to go and get one and send in.
Get character references, volunteer letters if you have done any as volunteering is highly considered in canada, make sure all copies are notarized.
Read very carefully so not to make any mistakes, you can always call immigration and speak with an agent if you not sure of something. Always go to the experts.