These are the most common chords in the key of D.
The TONIC chord, based on the first note of the scale, also called the one chord, is D.
The SUB-DOMINANT chord, based on the fourth note of the scale, also called the four chord, is G.
The DOMINANT chord, based on the fifth note of the scale, also called the five chord, is A.
When a note seven scale-steps above the root of the triad (three-note chord) is added, a "7" is added to the chord name. Seventh chords are more dissonant/tense than simple triads, and they lead our ears/minds to expect a less dissonant chord to follow. Dominant seventh (V7) chords usually lead back to the I chord. (Notice that Roman Numerals are often used to designate chords.)
Note: Any of the two- or three-chord songs you have learned in the key of C can be transposed to the key of D by moving the chord name up one whole-step. More to come...