How do I use my Eurail pass?

Depends on which countries you want to visit and which trains you'lld like to take. 

The pass= ticket. Seat reservations are separate from the pass, and app and require an extra fee per seat (although you can minimise it in a lot of case, even bypass them If you have a lot of time). 

State your travel plan, and we can help. 

Spain, France (and a little less Italy) are quite reservation heavy countries. 

In a nutshell, there are

• Reservation free trains (Most trains in Europe, mostly local and regional services. S, L, IR, RE, RB, Sprinter, R, TER,… many different names)
• Trains with optional reservations (Long distance trains in Germany, Austria, and the UK; IC, EC, ICE, RJ,...). You can board any train you’lld like and find a free seat, but if someone shows up with a reservation you’ll need to find another seat. Advisable to reserve on busy travel days, like summer-weekends, long holiday-weekends, or when travelling in a small group…
• Trains with mandatory seat reservations (Nighttrains, TGV in France, Eurostar, Thalys, Frecce-trains in Italy, AVE's in Spain). Usuallyhigh speed, long distance (sometimes International) trains. Without a seat reservation, you don’t get on the train. These trains can sell out, sometimes weeks in advance (Eurostar).