Hi, and welcome to Pianostreet!
Start with easy music, as it can get overwhelming if you are new to this. The approach can differ depending on the style of music and transposition. There are also techniques that can be employed for specific types of transposition. For example: when sight transposing a concert pitch score on B-flat clarinet, I often like to read the treble clef as alto clef.
Start with transpositions that you may commonly encounter. +/- one or two semitones.
For practice repertoire you can begin with:
Hymn/chorale tunes in unison (monophonic texture). Play melody both hands in unison. Play octaves if you so desire. You might also want to do this as LH solo.
Opera vocal score piano reduction (homophonic texture). There can be a lot of repetitive accompaniment patterns. It can allow you to build ability in looking ahead, since there is a lot of time spent repeating patterns. It also enforces being mindful parts that you do not play on the piano.
Early Baroque keyboard music or easy Bach (polyphonic texture).
Four part hymns and chorales (homorhythmic texture). Start with hymns, then when comfortable move on to the Bach chorales.
Open vocal score, hymns and chorales (homorhythmic texture). Start with 4 staff, treble and bass clefs. Then you can move on to doing the same with C clefs.
Open vocal score, Renaissance (polyphonic texture). Start with a modern score with treble and bass clefs. Move on to reading with each part on its own C clef. For example, music of Palestrina, Josquin, Lassus, and Victoria.
For a challenge you could do, Bach BWV 1080 in open score.