In plain words
If slain, you will attain heaven; if victorious, you will enjoy the earth. Therefore arise, son of Kunti, resolved to fight.
Word for word
Slain, you will attain heaven, or having conquered, you will enjoy the earth; therefore arise, O son of Kuntī, resolved upon battle.
The commentators
Bhagavadgītābhāṣyam
युद्धे पुनः क्रियमाणे कर्णादिभिः —
हतो वा प्राप्स्यसि स्वर्गम् , हतः सन् स्वर्गं प्राप्स्यसि । जित्वा वा कर्णादीन् शूरान् भोक्ष्यसे महीम् । उभयथापि तव लाभ एवेत्यभिप्रायः । यत एवं तस्मात् उत्तिष्ठ कौन्तेय युद्धाय कृतनिश्चयः ‘जेष्यामि शत्रून् , मरिष्यामि वा’ इति निश्चयं कृत्वेत्यर्थः ॥ ३७ ॥
In plain words
And once the fighting is actually joined, with Karṇa and the rest in it —
"Killed, or — you will win heaven": that is, if you are killed, heaven is yours. "Or, having conquered" Karṇa and the other champions, "you will enjoy the earth." Either way, all you stand to do is gain — that is the point. And since that is how it stands, "rise, son of Kuntī, resolved for battle" — meaning: settle it in your mind, "I shall beat these enemies, or I shall die." || 37 ||
Close to the Sanskrit
But when the battle is being waged by Karṇa and the others —
"Slain, or you will attain heaven" — being slain, you will attain heaven. "Or, having conquered" Karṇa and the rest, heroes, "you will enjoy the earth." In both ways alike there is only gain for you — such is the intent. Since it is so, therefore "rise, O son of Kuntī, resolved for battle" — having made the resolve "I shall conquer the enemies, or I shall die": this is the meaning. || 37 ||