Sossusvlei/Sesriem are the better part of a day's drive from Windhoek. There are some rather difficult mountain passes on some secondary roads, so you'll have to choose your route wisely if you don't bring a high-clearance vehicle there are several routes from Windhoek. The best route without nasty mountain passes would be from the south-central town of Mariental. On the other hand, the passes provide excellent viewpoints.
Several tour companies operate tours from Windhoek and Swakopmund to Sossuvlei, so then you don't have to worry about the driving at all.
Sossusvlei is inside Namib-Naukluft National Park so a permit is required to enter. Permits can be purchased at the park office just inside the gate at Sesriem. The gate opens at sunrise and closes at sunset.
