Where are you going? My travel usually takes me out of the country. To prepare, I google maps, hotel options, climate, places to visit like museums, great buildings, local festivals and markets. I'm looking for days and hours things are open, what local things are available to buy. I try to learn some useful phrases or refresh my knowledge of the language. I watch the weather online for my destination and notice if it is following historical trends.
If you are living near a large city, you may find a meet-up for travelers. You could attend and get tips from their members who have visited your destination.
I try to pack for the expected weather, but always include something in case the weather becomes extreme. So a pair of shorts and a very heavy sweater or silk underwear give me a cushion for the unexpected. I try to make my wardrobe match the typical clothes of my destination. A nice pair of comfortably fitting jeans are a safe bet almost everywhere. You should break in any shoes you plan to walk or hike in before your trip. Be sure the socks you bring work with the shoes. If you are planning to spend any time in shared shower, for example in a campgrounds or hostels, flip flops are a must.
Be sure to use the information you found online to create an itinerary.