There have been a whopping seven Marvel films between the first Ant-Man and its sequel, Ant-Man and the Wasp, so if your memory of where you last see Scott Lang is a little hazy, you're forgiven. As with most things MCU, timelines can get a bit complicated so we're here to help get you up to speed.
Ant-Man Meets Falcon
In the original Ant-Man, Scott finds himself briefly fighting with Sam Wilson (aka Falcon) at the Avengers' Headquarters while trying to steal a piece of technology to assist in their heist. The film's after-credits scene actually takes place shortly after the beginning of what would be the next MCU film, Captain America: Civil War. Steve Rogers and Sam have the Winter Soldier (aka Bucky Barnes) captured and are unable to call in for their usual help due to the Sokovia Accords. Sam tells Steve that he thinks he knows a guy who might be able to help.
The Airport Battle
Moving into the actual events of Civil War, Steve and the rest of his crew including Sam, Bucky, Hawkeye, Scarlet Witch, and Scott are brought to Germany after setting out to apprehend Barron Zemo. They are intercepted by Tony Stark and his team at the Flughafen Leipzig/Halle Airport where a massive battle ensues. Scott adds several unique abilities to the fight, as many of his skills are yet to be known by the rest of the group. We see him and Cap throwing a miniature truck to then enlarge it, squaring off against Black Widow by shrinking down, using his increased strength, and riding one of Hawkeye's arrows towards Tony Stark to slip beneath and disrupt his suit. Finally, Scott shows off a new power — the ability to increase in size becoming Giant-Man.
Unfortunately for Cap's team, all are apprehended — except for Steve and Bucky, who flee in pursuit of Zemo and the truth. Those captured are taken to a floating prison called the Raft, which is designed to hold people with exceptional powers of some kind. Thankfully, Steve returns to set everyone free at the end of the film.
Leading into Ant-Man and the Wasp, we aren't 100 percent sure what Scott has been up to outside of reconnecting with Hank Pym and Hope Van Dyne to attempt to further understand the Quantum Realm. What we do know is that Ant-Man and the Wasp is set before the events of Avengers: Infinity War, and could contain clues pointing towards potential solutions to the ending of that film.