OK you got some good advice and some lets say some questionable?
Since your new I'm gonna say to not take all advice (even mine) as gospel, do some reading and research on your own. If some one gives you advise without explaining why, ASK! understanding the principal behind the advise will help you learn how to grow better
If your starting with bagseed to get your learn on then mixing cheap soils with your happy frog will be ok, you'll "learn" not to and it won't cost much. If you spent $100 on 10 seeds why would you skimp on the soil? You can totally make your own soil mix but by the time you buy all the ingredients and mix them you will have spent more than if you buy a good soil blend Fox Farms is good soil and best is to go light warrior for seedlings, happy frog for week 3 to 5 and ocean forrest for flower. You will find that mixing them is good as well
My final (for flower) soil mix is (if you read my noob advice thread you'll see I got this from a tomato forum)
one part happy frogs
two parts ocean forest
a handful (cup) of sphagnum moss
and one tablespon Dolomite Lime (per gallon of soil)
The reason I said not to add perlite to your happy frogs is because it retains water and HF's has bark and moss in it that also retains water,,,,
THE BIGGEST MISTAKE MADE BY NEW GROWERS IS OVER WATERING
So my advice is not to add more water retaining material to your first grow, learn to read the soil before you begin to experiment, and I'm all for experimenting
For the same reason (as above) do not put your seedlings into 3 gallon pots, you increase your possibility to OVER WATER 10 fold the seedling can not drink that much water and it will take longer for the soil to dry out, increasing your chances for drowning your plant and possibly mold (as well as other problems)
+ rep to the guy that said put your seeds in a small cup of tepid (room temp) water, if tap water let it set out 24 hours to evaporate the clorine. I go 12 to 24 hours till the seeds sink then put them in soil 1/8 to 1/4 inch deep do not pack the soil when you cover it. Lots of folks use the 16oz red party cups (I have too) these are good for 3 to 4 weeks. I use 6 inch coir pots because they are good for my total veg period (5 weeks) and you just plant them when time to go in a bigger pot (no messing with roots)
Now I'll add a tip/trick buy a small bag of seeds (any seeds, I like tomatos) from home depot, lowes walmart and put them into the cup your going to soak your seeds in (to be clear fill cup (1/2 cup to a cup) let set for 24 hours, add entire bag of tomato seeds, let set for another 24 to 48 hours, remove tomato seeds (throw away or plant) put your seeds in 12 hours, plant em, use the water in the cup to water them
Why, (you can google seed soak water) all seeds have everything to start things off in them there is an enzyme that starts the process when they get wet and if you put lots of seeds in water it gets enriched with beneficial stuff seeds like this gives your "good" seeds a better healthier start as they enter our world
nuff for now
Good Luck