Well, if you want to know about amino acids specifically, I'd start with organic chemisty. IIRC Khan Academy teaches organic.
To give you the short answer to what an amino acid is, it's… wait for it… both an amine and an acid. You see that nitrogren with the two hydrogens on it in your pick? That's an amine group, meaning chemicals that have that attached are called 'amines'. Amines are alkaline. If you put them in water they accept a hydrogen from a water molecule, leaving a free OH- ion in the water. See that carbon with the two oxygens and the hydrogen on it? That's an organic acid group. When you put it in water, it gives off an H+ ion, just like any other acid. Any compound with one of these attached is an 'organic acid.'
Amino acids are both acids and bases at the sample time. This means under the right conditions, the amine group of one amino acid can bond to the acid group of another amino acid. This is called a 'peptide' bond, and the resulting compound is a called a 'peptide.' If you start making long chains of these, eventually they will start doing weird things with themselves, like coiling into helices and bending back and forth into sheets. These very large peptide chains are called 'proteins.' They have many, many different functions in the body.
Feel free to ask any more questions, but like I said, learn a little organic chemistry, then start getting into the biology side of things.