To get to the Onuma area, there are two options
1: Drive. Typing "赤城山" (try あがぎやま or あかい + しろ
2: There are buses from Maebashi (Japanese only). These will also take you to the visitor's center.
Of course, you can use any of the parking areas if you drive. The Onuma-hiroba lot is large and is centrally located between the two trails up the mountain.
I recommend doing the loop in the same order I did - up the trail on the left (more or less the north) and down the trail on the right. The left trail is steep, rocky, and at least when I climbed it (early May after two days of rain) pretty darned muddy. The ridge hike from Kurobi-san to Komagatake was wonderful, with excellent views and without much up and down. It is better, however, to hike it from left to right (north to south) because the better views will be in front of you rather than behind. The trail down from Komagatake to Onuma is in large part stairs, making the descent quite easy. Total hiking time is about three hours plus however long you spend enjoying your lunch at the top and snapping pictures. 4:00 - 4.30 would be normal for a typical hiker.
As for weather, there were a few patches of snow during Golden week (and the associated mud), but nothing one would need special equipment for. Overall, it is a pretty easy climb. Note that at Onuma, there are some noodle and trinket shops, but not much will be open if you arrive early in the morning. There are plenty of vending machines, but you need to bring your lunch.
Kurobi-san from Akagi Jinja