Backpacking for something new ahead may be a bit confusing. You need everything yet you can't make your rucksack/backpack heavy So, I guess I can answer it correctly. I had a similar situation before my first trek.
The very first thing you must check before going to a trek is the temperature at night. Trekking through the day might make you feel sweaty, what most people miss out on is the need of warm clothes at night. No matter where you trek on mountains you will see a sharp temperature fall at night. Pack clothes that occupy less space. Try to carry clothes as less as possible. I just had a warm jacket, a t-shirt and a hand towel in my bag on a one night trek.
2) Rain cover / Poncho
Rain and hailstorms are one of the most unexpected things on mountains. Make sure you carry something that can cover you and keep things wet. I would suggest carrying a poncho as it can cover your both you and your bag, its easy to wear and fold.
3) Eatables and water
Avoid carrying any food items on the top if their are any cafes over their. Food occupy a lot of space and adds unnecessary weight. Even if you need to carry food, never carry food that can be spilled. Carry items that are dry, packed and easy to eat anytime. Plenty of water should be with you. Without water one can easily get dehydrated during trek. Water is also something you won't easily find on your way so preserve it and refill wherever possible. Do carry a small garbage bag to put all of the litter as you may not want to make the trail dirty.
Take a small pouch of basic medicines for headaches, common cold, and my minor cuts. There often comes a situation you start to feel drowsy, uncomfortable and your body temperature suddenly rises. Taking a few medicines and band aids won't take a lot of space but can come really handy at times.
5) Torch, sanitizer and more..
As night approaches, light would be something you will be looking for as the mountains gets really dark in the night. Never ever forget to carry a torch with you on a trek and also make sure to carry a spare battery. Along with this, sanitizer will keep you clean as water is difficult to access and obtain. Along with these usual stuff take a few paper clips, some polybags, rubber bands and a waste cloth. These things are small but proves to be very useful at times.