If you’re just starting out, I would go with a lower carbon steel - in fact, I would practice the shape and design of a dagger on some mild steel first. Mild steel is the cheapest and most widely available.
Mild steel can harden a little, but it won’t serve a real great purpose if you’re putting the tool to work.
For that, in your situation I would probably find some leaf spring or coil spring from a car - this steel can be hardened and hold a good edge and is typically cheap and easy to find. I’ve gone to a junk yard and grabbed some coil/leaf springs.
You can also purchase steel online at places like http://newjerseysteelbaron.com/ which will allow you to get the steel closer to the shape you need, means less forging to shape. W-1 and W-2 are both good choices, if you go the 10 series of carbon steel I would go 1084 or lower as it’s more forgiving in heat treatment than 1095.