Just use the reagent to learn the first trait. So, the same way you would use a food or drink item.

You should be able to make the lowest level potions as long as you have the water and two reagents that go together. As you level, you will need to increase the Solvent Level with a Skill Point. You can go to 10 with no points in the line.

Not all reagent combos make something, or make something useful. Later on you will be able to add a 3rd Reagent to get potions with 3 effects. These are known as Tri-Pots. These are the ones that can restore all 3 attributes or an attribute and give you extra power and regen or whatever.

This will tell you what effects the reagents have

http://tamrieljournal.com/crafting-and-professions/alchemy/

Note, learning a new effect when making a potion gives you a bunch of Inspiration. I don't think eating it gives the same, not sure.

That's very useful.

Thanks for the advice there Nestor.