Hiking, like everything outdoors, requires a little planning to guarantee a great time. Depending on where you are hiking, the duration of the hike, the time of day you are hiking, and members of your group will determine exactly what you are going to take. Safety is always first on the list for hiking.
I always carry an abbreviated first aid kit with me. Plenty of water is next on my list. A map of the area is also necessary. I usually carry a bear whistle, waterproof matches, a flint stick, a flashlight, my identification, a signal mirror, compass, and something for bugs - a mosquito bracelet or wipes are a great idea.
I usually wear convertible hiking pants. These are pants that can easily be converted to shorts by simply unzipping the legs, or vice versa. A tech shirt is preferable to cotton. If it is chilly in the morning a light jacket is good for layering. You can find jackets that double as a vest when sleeves are removed. Or a vest under a light jacket is good. Carrying a change of clothes is important for extra warmth or if you get soaked by rain or fall in a lake or river. Hypothermia is a real concern when hiking.
If your dog is hiking with you, you need to prep her for hiking as well. There are plenty of dog hiking packs available. I have my dog carry her own water. It is important to train and/or test your dogs comfort with drinking from a water bottle or carry a collapsible bowl for drinking before your first hike with her.
Checking the weather forecast is important but packing your backpack for the unexpected weather and an unexpected accident is just as important. The better you pack and plan the more at ease your hike will be and the more fun you will have.