Alright its your first grow, and if its in a good guerilla spot your going to have a bit of a hike ahead of you.
Kens the master about these parts when it comes to make shift greenhouse's etc. Listen to him, Speaking of that I need to pick his brain on the matter :P.
My advise when it comes to amending your soil is Gauno. Gauno makes soil golden. If its good grassland soil that has plenty of nitrogen and organic material then dont worry about adding in potting soil your plants will grow fine for the most part.
*Gauno is high in Nutes your plant needs in both Flower and veg, which is why I recommend it as a simple additive to grow better pot cheaply.
Yet again, Thats only if you dont want to lug in bags of soil.
Each one of your plants should have a 2x2x2 hole minimum of tilled soil each. So investing in a good ol' fashion army hiking shovel is a great choice.
You want to atleast let the seeds sprout up at your house if you can, if you cant then build a little structure, In my experince the cheapest is a makeshift stick green house with some thin plastic like used on hoop houses etc.
Chobble