StarCraft II: Heart of the Swarm Terran Build Order Guide

10 Supply Depot

12 Barracks

15 Refinery

16 Marine

17 Orbital Command

17 Supply Depot

18 2x Marine

20 Reactor on Barracks

20 Command Center at natural expansion

21 Factory

Once Reactor is done, make several Marines
Once Factory is complete, build Starport
Once Starport is partially built, make 2x Widow Mines
Once Starport is complete, build 1-2 Medivac
24-26~ Refinery

Once Medivac is complete, drop Marines into enemy mineral line
Place Widow Mines either defensively or near enemy base entrance depending on how aggressive they are

Terran are in a similar spot in Heart of the Swarm as they were in for Wings of Liberty, but Medivac drops are much more effective and Widow Mines can be extremely potent. For this build, you aim to get an early expansion and build your tech to support a Marine/Medivac/Widow Mine build early on. Marines can be used as your frontline and as a useful harassment tool with Medivac drop potential. Meanwhile, the Widow Mines add a sense of discomfort to your enemy.

The main goal with this build is to establish a solid economy that can build enough units to bot defend the base and harass with Medivac drops. The key to winning is timing and location with said drops, and that simply takes practice.

Once you've completed the above build order you can transition into a Marauder army if the enemy has armored units, and go with the standard MMM build with Widow Mines for good measure. Stick to biological units and expand to an early third base if the enemy isn't aggressive. Otherwise, saturate your mineral lines, constantly pressure with drops, and place Widow Mines in locations that force the enemy to be afraid of attempting mobility.