How and where can I propose new MTG cards?
Wizards of the Coast employees are not allowed to look at unsolicited card designs, from the Wizards of the Coast FAQ on Idea Submissions:

Game Submissions
We do not review unsolicited game or card submissions, return submissions, or make comments on submissions we haven't reviewed.

If you want other people to look at and critique your designs you can go to places like MTG Salvation's Custom Card Creation forum, but the only way to make actual cards is to work for Wizards or submit your design for something like You Make the Card