English only adds this phrase to infinitive without to. All are the same.  Learn imperative for -are verbs. Drop ending add iamo. Example : (noi) parliamo  Learn imperative for -ere verbs. Drop ending add iamo. Example : (noi) ridiamo  Learn imperative for -ire verbs. Drop ending add iamo. Example : (noi) partiamo For example dare (to give) makes diamo (let us give) or avere (to have) becomes abbiamo (let's have).
Note that this is equivalent to "Let us" in English. Note also that this is easiest to learn. Follow the same pattern even for irregular verbs.