Pedro,
The knifemaking community is a great place to be ! The lengths that makers go to in assisting others never ceases to amaze me.
When brand new myself I got a ton of help and similar advice. I tried a couple paint stick knives but found it only helped to understand "grinding theory" in it's most basic form.
I found quickly I could grind a paint stick into a BSO (blade shaped object) in one or two passes per side and I was missing out on the learning to grind actual blades.
Grinding a blade is a MUCH slower process.
It wears out belts
It gives the oppertunity to make mistakes and begin to understand the causes and effects of grinding
It teaches you that you WILL burn your fingers while grinding and to keep your blade cool by dipping in your swarf bucket.
It goes slow so you've got time to figure out how to FIX your grinding mistakes at the grinder
It forces you to make many "approaches" to the belt with your blade. It takes practice to get on the belt without gouging either your edge or grind line.
Happy with it or not you'll end up with an actual blade that can be heat treated and made into a complete knife.
Keep in mind that grinding is just one aspect of knifemaking. Those butchered up first blades will come in handy when figuring out things like-
I want to heat treat my own blades ????????
I want to etch in ferric
I want to start using a makers mark and NEED to experiment to figure out how
I want to grind a nail nick into a folder blade
I want to experiment with a tapered tang / bolsters / guard / etc. etc. etc.
For all these reasons I'd recommend that you pick up some carbon steel or 440c stainless to start with. Wether you'll be heat treating yourself and the tools you have available SHOULD determine your steel choice.
Steel costs money ! Thats just a fact of knifemaking. Things that also cost money- electricty, abrasive belts, cutoff wheels, drill bits, hacksaw blades, et. etc. etc.
As a beginner low cost blade quality known steel is the way to go IMHO. You'll be suprized that you can get blade steel for roughly the same cost as getting mild steel from Lowes/Home Depot etc. so why waste your consumables "practicing"
Shipping steel is spendy. When you order steel try to get enough to make the shipping cost effective.
Don't get crazy and order a ton of steel though, LOL. As you progress your taste for steel will likely change with your skills and tools. 2 or 3 8 foot sticks of 1 1/2" x 3/16" is alot of steel and will likely keep you busy for a couple years if need be. (depending on alot of factors- mainly how much time you spend making knives)
Just some things from my experience that may help ya out a little or get you thinking about the stuff that happens when the grinding is done
Take care- Josh