How can I learn about and keep track of major Magic tournaments in my local area?
The official M:tG site offers a store and tournament locator. Just enter the name of the town you want to look up, and it will show you upcoming, official tournaments in that town and in an area around it.

Of course, this can only cover sanctioned tournaments that WotC knows about - for unofficial tournaments, your best bet is probably to get in touch with the local player base and store owners.