Level Legerdemain and you can really reduce the Stamina Costs related to this. There are Medium Armor passives that help with Sneaking, both cost and detection. There are also Champion Constellations that help with the Stamina Costs

There are some dropped sets that help with Sneak, but they don't drop until higher levels. Nightshade and Shadow Walker are two of them.

Nights Silence, a crafted set that only requires knowing two traits on each piece you want to make, eliminates the speed penalty so you can sneak faster.

However, until you can get those sets, or make those sets or unlock those passives, use Invisibility Potions or learn how to leverage line of site. For the latter, as long as there is something between you and the NPC, you won't be seen.