Diminished chords are formed by combining the root, flatted third (minor third) and flatted fifth of the major scale (1, ♭3, ♭5). For example, the notes of the C major scale are C, D, E, F, G, A, B. To form C diminished, you combine the notes, C, E♭ and G♭.
A quick way to form this chord is to hold the minor chord, for example C minor, and to flatten the fifth note of the scale, which is G. Just play a regular minor chord but instead of playing G, play G flat. Alternatively, you can play a major chord, C-E-G and simply flatten the third and fifth notes of the scale, resulting in C-Eb-Gb.
To learn more about this chord and others, check out my course, Piano Chords: How To Form Basic Chords On Piano And Keyboard.
On your piano keyboard, to form any dim chord, simply hold the root note and each time skip two keys and play the key to the right of the two keys you skip.
The formula for the diminished chord is root plus three half steps plus three half steps (Root + 3HS + 3HS).
Symbols for this type of chord are dim and °. For example, Cdim and C°.
In most sheet music books, Cdim or C° denotes a diminished seventh chord with C as the root. In this case you add the double flat seventh (♭♭7) of the scale. For example: C-E♭-G♭-B♭♭). Bbb (B double flat) is enharmonic to A on your piano but in theory the correct name for the note is Bbb. The note, A is in theory the 6th tone of the scale.
In other cases, mostly in modern jazz books and some music theory literature, when you see the chords, Cdim or C°, you simply play the triad and leave out the seventh. You play the seventh when you come across the chords written like this: Cdim7, C°7, or Cm6(♭5). 
Diminished Chords Chart
- Cdim – C Eb Gb
- C#dim – C# E G
- Dbdim – Db Fb Abb (G)
- Ddim – D F Ab
- D#dim – D# F# A
- Ebdim – Eb Gb Bbb
- Edim – E G Bb
- Fdim – F Ab Cb
- F#dim – F# A C
- Gbdim – Gb Bbb (A) Dbb (C)
- Gdim – G Bb Db
- G#dim – G# B D
- Abdim – Ab Cb (B) Ebb (D)
- Adim – A C Eb
- A#dim – A# C# E
- Bbdim – Bb Db Fb (E)
- Bdim – B D F
There are only three diminished 7th chords. Every dim7 chord is really four chords containing four evenly spaced notes each.
1. Cdim7, Ebdim7/D#dim7, Gbdim7/F#dim7 and Adim7 all contain exactly the same notes. It’s really four inversions of the same chord containing the notes C-Eb-Gb-A.
2. C#dim7/Dbdim7, Edim7, Gdim7, & Bbdim7 all contain exactly the same notes. It’s really four inversions of the same chord containing the notes C#-E-G-Bb.
3. Ddim7, Fdim7, Abdim7/G#dim7 & Bdim7 (Cbdim7) all contain exactly the same notes. It’s really four inversions of the same chord containing the notes D-F-Ab-B. 
Diminished Seventh Chords Chart
- Cdim7 – C Eb Gb Bbb (A)
- C#dim7 – C# E G Bb
- Dbdim7 – Db Fb Abb (G) Cbb (Bb)
- Ddim7 – D F Ab Cb (B)
- D#dim7 – D# F# A C
- Ebdim7 – Eb Gb Bbb (A) Dbb (C)
- Edim7 – E G Bb Db
- Fdim7 – F Ab Cb (B) Ebb (D)
- F#dim7 – F# A C Eb
- Gbdim7 – Gb Bbb (A) Dbb (C) Fbb (Eb)
- Gdim7 – G Bb Db Fb (E)
- G#dim7 – G# B D F
- Abdim7 – Ab Cb (B) Ebb (D) Gbb (F)
- Adim7 – A C Eb Gb
- A#dim7 – A# C# E G
- Bbdim7 – Bb Db Fb (E) Abb (G)
- Bdim7 – B D F Ab
You will notice above, that some notes have different names. These are called enharmonic equivalents. For instance C♭ and B are enharmonic equivalents. On the piano, the same note is played but in music theory the note name depends on the key the music is in. Where you see a double flat (for example Bbb), simply take the note which is already flat and make it a semitone lower. Bbb corresponds to the A key on the piano. Learn about enharmonic equivalents here and the difference between piano notes and keys.
How to Play Diminished Chords on Piano (Video):
If you’re just starting out on the piano, don’t worry too much about more ‘advanced’ chords like the diminished ones presented above as yet. These chords are not very common. More common chords are major and minor chords. Start with these. Later, you can move to the diminished chords. Actually, while they seem complicated, they are not. In time everything will piece together.
Here’s the main piano chords page for info on all kinds of chords.
PianoForAll is the best online course I’ve found on learning to play the piano. Check it out here.