Make no mistake about going to The Sequoias in the summer; it is very hot. Even at high elevation, it will fluctuate between the high 80's-100's degrees.
Be sure you have water--preferably from a reusable bottle--and if you find a way to make and keep it ice cold, you will appreciate it once you start exploring the different areas of the National Park. Bring snacks if you feel so inclined or just eat a filling, nourishing breakfast before you start your day.
Wear comfortable shoes and breathable clothes made for warm to hot weather.
Bring and apply good and effective sunscreen. Sunglasses 😎 or a good sun hat are great accessories to also have.
If you love taking pictures, definitely take a camera because there are many scenic photo spots: majestic trees, rivers, massive rocks, canyons and ravines.
Hiking is plentiful with varying degrees of length and difficulty. Definitely do your research so you can find the ones that suit your fitness level. Here's an example of an article guide with some suggestions on hiking trails: