the conjugation of regular -er verbs is
je: -e
tu: -es
il/elle: -e
nous: -ons
vous: -ez
ils/elles: -ent
"Je danse" is already in conjugated form for "je" with the infinitive being "danser"
translating to: I dance
Je jouer would conjugate to "Je joue" since it is an -er verb
translating to: I play