The easiest and first place that you can get this, is on the first planet that you visit, Eos.
Following the main story mission, A Better Beginning and completing it by activating the Remnant Vault, will net you 50% viability to the planet.
To get the other 50%, you need to do side missions and setting up forward outposts.
Each side mission is worth 10% viability and each forward outpost is worth 2%
Side Missions that you can complete include:
The Secret Project - Started by finding the datapad at Site 1, when you first land on Eos
Shock Treatment - Started from your settlement, Prodromos
Making an Impression - Started after you have left Eos, obtained your 6th squad member and then returned again after the atmosphere has completely cleared out. Given out by Hanely in Prodromos
Defeating the Kett - Started after you have left Eos, obtained your 6th squad member and then returned again after the atmosphere has completely cleared out. You can also start this by finding the Kett stronghold on the western side of the map and attacking it.
There are other side missions that do become available, but these are the main ones. Doing a combination of these plus the forward outposts will net you the 100% planet viability.