Hi Rajath, A trip to Himachal Pradesh sounds exciting! You can read about trekking essentials here: https://goo.gl/dqTBJX If you plan on hitchhiking you can consider carrying some camping gear too. You can read about it here: https://goo.gl/j874dU You can read about beautiful stories from our travellers who have explored several amazing places in Himachal Pradesh here: https://goo.gl/ZWvYRD
Every article has a map and location linked to it. If you need a physical map, visit any tourist office in the area. Most of the hotels or hostel lobby have some stacked in the reception. Happy Travelling!
For backpack I'd ask you to carry sleeping bag if you have and also yoga mat. one jacket. and as least clothes as you can take. because there'll be times when you have to walk a lot. and since you are going on mountains, you might have to trek a lot. also take safety things like pepper spray or a taser if you have. might help in worse case.
rain coat and big polythene.. may be as big as you . or bag cover you can also read my article how i did my Delhi-Leh trip. https://www.tripoto.com/trip/leh-couldn-t-have-been-597823ff239db