It depends upon the edition.
More recent editions of D&D such as 4th and especially 5th have rules for combat that are very heavily positional, indeed people say now it is nearly half a war game.
If running an earlier edition you shouldn't have a problem since combat basically just involves noting distance from enemies which is fairly easy.
More generally the one thing you will need is a way to stat npcs or look up the stats of npcs on the fly. This is one of the things that has I confess put me off idea of trying gming myself, since I don't know any rpg system quite well enough to get the numbers correct to quickly come up with what various monsters etc we need.
That being said, for the games I've played very extensively our gm has had the correct programs on his laptop, eg, hero lab for when we were playing mutants and masterminds, and would stat the major npcs before the session and have instant profiles for miner ones he could pull up, indeed he had a lot of npc stats stored so that he could fairly easily pull a villain or two he thought the players were having it a little too easy .
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)