Would I be able to get a Start soil( the organic soil alone, before mixed with organic nutrients ) at a store like Home Depot or Canadian Tire?
Aim for an organic potting soil. Black Gold is acceptable. If you can get to an Ace Hardware they have some better selection. Most places have plant nurseries near by (people need trees and vegetables and shit) and you can get some good stuff there. Rose potting soil is usually well balanced and works well for flowering plants like cannabis. In addition to soil you will need to purchase a fair amount of perlite. I'd suggest enough to mix 2 parts of soil with 1 part perlite, even if the potting soil already contains perlite. This will add much needed air to the root zone and reduce the risk of over watering. Another suggested addition is Crushed Dolomite Lime (available everywhere). This will help maintain your pH and provide a slow release of calcium, magnesium, and many other micro-nutrients.
Or any pre mixed organic soil that would be able to grow from seed to harvest with?
Here's an idea:
Select three different soils. Look at the nutrient concentrations listed on the labels. Select a soil with low levels of all nutrients for your seedlings, almost totally devoid is best as less is more during this phase. Grow them in party cups. Then use a higher nitrogen soil to transplant into 1 or 2 gallon planters. Finally finish them in 3 gallon or 5 gallon planters using a higher phosphorus and potassium potting soil when you transplant from there.
Also I was thinking of making the top half of the soil where the seed will go in less powerful, the bottom half will be a more powerful soil mixture so when the roots get bigger they will reach the stronger soil mixture to allow the plants to get the nutrients they need during flower.
Bad idea. The tap roots reach down before reaching out and they will hit that hot soil and burn up. I've tried it myself and it doesn't work. You want a balanced and even soil for your roots to grow in. If they get hungry then we can talk about a top dressing you can water through to increase the nutrient levels.
And, how how often do I have to water the plants?
Water your plants whenever you need to
lol. Seriously though, you want to add enough clean water through the soil to get 25% of it to drain out the bottom. With organic soil and just water, run off is less important... but still a good practice to have. You will need a way of managing this run off that doesn't involve jostling the plants too much.
Once the soil is saturated you want to wait until it is nearly dry before watering again. Not completely dry, but almost. Test this by lifting the planter and checking the weight. It will get really light when you need to water again.
Always allow your water to sit out for 24 hours before using it to allow any chloramines to off gas from the water.