Assembled on the field of dharma, at kurukṣetra, desirous of battle, What did my (sons) and the sons of Pāṇḍu do?
धर्मक्षेत्रे - in the field, on the field (neut., locative, sing) of dharma
कुरुक्षेत्रे - in the field, on the field of the Kurus
Two locatives together are called a sati saptami or a locative absolute. It gives an indication of 'when'. Hence this verse if often translated 'When in the field of ....'
समवेता - assembled, come together (masc, nom., plural past participle of sam+ava+√ i). The actual word is समवेता: but the visarga is lost by the rules of sandhi before the य of युयुत्सवः
युयुत्सवः - desiring to do battle (masc., nom., plural, adjective; desiderative of √yudh)
मामकाः पाण्डवाश्चैव = पाण्डवाः च एव
मामकाः my, mine (masc., nom., plural)
पाण्डवाः (lit. Pandus - sons of Pandu)
च and (avyaya)
एव indeed, only (avyaya) here used as a rhythmic filler
Note that मामकाः does not lose it's visarga before the pa of पाण्डवाः
किमकुर्वत संजय = किम् अकुर्वत संजय
किम् - what (interrogative)
अकुर्वत - (they) did (3rd, plural, past ĀP √kṛ)
Anything underlined - just means we will learn it at a later stage.