All the places you are visiting are major tourist centres. Lots of people travel alone just use common sense and keep your valuables on your person - never in your luggage. If you stay in hostels you are likely to meet people of your own age. Research the hostel and find out how easy is it to get to the city centres and back and read the reviews carefully. Read all the sections on this site about reservations as you’ll need them between Amsterdam and Paris and Paris to Lyon if using Thalys or TGV trains.

DO note that on the days right after Natale=Xmas and around Nuevo Ano=Nw Yr, trains can be very full. Staying in hoStels (with multi-bed rooms, but many also offer private rooms) will usually bring you in contact with many such travellers from all over the world doing about the same. Some hoStels also organise special outings and sightsee with the group. Check also sites like booking or hostelworld.com for that. Buene viaggio!