सौमित्रे O Lakshmana, वनम् to the forest, प्रहिताः having been sent, अद्य today, इयम् this, वनवासस्य exile in the forest, पूर्वा first, निशा night, उत्कण्ठितुम् longing in your mind, न अर्हसि does not behove you, ते भद्रम् wish you well.
O Lakshmana, this happens to be the first night of our exile. You should not have any longing in your mind. Wish you well