If you want to learn how to make chords, the best place to start is the major scale. Take a look at this scale - it's the C major scale.
You can make chords by combining the different notes of the scale and playing them together.
If you take the 1st, 3rd and 5th notes of this scale (or any scale) you get a major chord. The notes here are C E G.
Each type of chord has a title and a formula or pattern. They are based on the major scale. You can read about what the titles mean on the Chord Titles page.
If you work off of a major scale you can make any chord. If you need to make a minor chord or a 7th chord, you follow the same process.
With a minor chord, you must lower the 3rd note of the scale by one semi-tone. So instead of "C E G" you play "C Eb G".
Note that you must change the notes of the major scale.
The same thing happens with a 7th chord. With this chord you must use the 1st, 3rd, 5th and 7th notes of the scale. You must lower the 7th note by one semi-tone.
The notes are C E G Bb.
Here's the step by step approach for how to make chords:
1. Look at or choose a root note.
Our example was C. It is the first note in the chord. It can include a sharp or flat.
2. Look (or think about) the scale for that root note.
You can see the major scales here.
3. Figure out what the chord title is telling you to do.
Learn the formula for that particular chord. Figure out what notes in the scale you should play.
You can find help here on the Read Piano Chords page also.
4. Apply the formula to the notes of the scale.
5. Play the chord!
Once you play chords for a little while you won't even have to think about all of this. You'll remember what they are easily and just play them. If you learn how to make chords you can eventually figure any chord out.
If you want a complete course in chords, check out Power Piano Chords. It teaches chords from basic to advanced to chord progressions.