suru vs shita vs mashou vs masen

する (su-ru) is "to do" (present tense)
した (shi-ta) is "did" (past tense, completed action)
しましょう (shi-ma-sho-u) is "let's do" (suggestion)
しません (shi-ma-se-n) is "will not do" (negative present/future)

More examples:

わかります (wa-ka-ri-ma-su) - I understand
わかりません (wa-ka-ri-ma-se-n) - I don't understand
わかりました (wa-ka-ri-ma-shi-ta) - I understood
わかりましょう (wa-ka-ri-ma-sho-u) - Let's understand

食べます (ta-be-ma-su) - I eat
食べません (ta-be-ma-se-n) - I don't eat
食べました (ta-be-ma-shi-ta) - I ate
食べましょう (ta-be-ma-sho-u) - Let's eat