You have to be aware that during your trip the sleeping bag will serve as your bed. In hostels you normally don't need one. On night trains it can be very useful.
Pay attention to the weight (less than 1.5 kilograms) and the packing volume (less than 10 litres). The temperature range of the sleeping bag should correspond to your destination. Of course, the purchase will depend on your budget.
Advantage of sleeping bags with synthetic insulation: they dry quickly if they get wet and are less expensive. Advantage of down-filled sleeping bags: the packing volume is small and they are light-weighted.
air mattress / sleeping pad
Decide between a traditional and cheap sleeping pad or a higher-quality self-inflating air mattress. The sleeping pad is more robust and you can also use it to sit down on the ground, for example when cooking outside. The expensive air mattress is of course more comfortable.
When travelling in the summer months, a simple and inexpensive tent bought at a discount for less than 50 € should be fine. And it will resist a rain shower. The most important factor is the tent's weight and volume of the package. Make sure that a two-person tent weighs around three kilograms.
Eating is an important factor for your trip, allowing you to either save or spend a lot of money.
Cooking on your own saves money, fills your stomach and is healthier. Supermarkets can be found everywhere, prices are almost the same throughout Europe. The best thing is to avoid the expensive supermarkets in train stations, rather looking for a discount store, sometimes a short walk away.
The typical homemade meal while travelling is noodles or rice with vegetables and sauce.
Important: compare the prices between a fast-food restaurant and when cooking by yourself. You will see that fast-food is very expensive.
For cases of "emergency", bring two packs of soup with you. You just need to add some hot water and you have a ready meal.
You can fill two large plastic bottles with water in your hostel. This should be enough for one full day. Everything else is available in supermarkets. Note the rules for alcohol in each country: sometimes there are special stores for alcohol only.
Recommend are a plate, a mug and a pot made of sheet metal or stainless steel. Add a fork, a spoon and a pocket knife. The pocket knife with additional can opener and corkscrew.
Use your shampoo. It cleans everything, even in salt water at the sea.
Inexpensive and sufficient is a camping gas stove for EUR 20, plus EUR 5 for a cartridge. These cartridges are available in all European countries. Caution: the gas cartridges are not allowed on aircrafts.
You should have small jars with salt and pepper or a spice mill with you. One or two plastic boxes for left-overs, open packs or vegetables and fruits can be useful.
Most hostels and campsites offer computers with internet access. Sometimes even for free. You can also find cheap internet cafés everywhere.
Internet is very helpful when it comes to booking your accommodation for the next few days and making reservations for trains or for finding train connections.
It is useful to have your own small and lightweight netbook with you. It allows you to store photos, listen to music and surf the net. However it is not absolutely necessary to bring one.