It will take wide–angle lenses and flat light to get the best photographs of the sequoia trees. You must also realize that if you go in the height of the tourist season, people will be in your pictures, so use these people to add scale and grandeur to your images. After all, without this reference to scale, your viewer will not be able to determine the size and mass of these giant trees.
The sequoia trees are difficult to photograph on bright sunny days. The contrast of the deep shadows and the bright highlights are too much to capture on your sensor. So look for cloudy days or get there early when the sun is lower.
Wide-angle to normal lenses work well in the sequoia grove, as well as a tripod and a bubble level.
The wider the better! These trees are very tall, so you need lenses in the 14-28mm range to have any chance of getting a whole tree in your image unless you stand far away. The longer normal lenses are good for photographing the trees and cropping the tops, placing people against the massive trunks to give a scale to their mass and not being concerned too much about their height.
Most of the time you are going to be pointing your camera upward. Expect a lot of distortion where the trees "bend" inward. You can use this to emphasize their height. If you want to have straight trees, you need to fix them later in an image-editing program or use a tilt-shift lens. These are lenses designed for architectural ...