Finding your platform
Most European stations will have all timetables printed out and displayed on boards around the station. They can usually be found in the main station area as well as on platforms. To use these boards you just look for your destination and then look across at the departure/arrive times and the platform number... simple. At the major stations you will also see airport-style computerised departure boards showing which trains are leaving next and from which platform, remember to look for the train's destination on these boards, it may not have the same final destination as you! The key thing here is to always allow plenty of time at the station before your train departs and if in any doubt just ask a station representative.
Boarding your train
Most of the European rail network runs like clockwork and it's not unusual for trains to not even be approaching the platform 2 minutes before the scheduled arrival time. They will quite often just appear with 30 seconds to go. They are equally as efficient at leaving the stations though so you don't have time to search for a seat as you wander alongside the train on the platform. For high speed, premium and some intercity services the platforms are labelled with carriage numbers and first and second class zones are also marked. You should try whenever possible to get into the correct zone, if you have a seat reservation your carriage will be marked on the ticket.
The main thing is to not panic, as long as you're attempting to get on the train it won't try to leave without you! Just try and be in the right place and get you and your luggage on with as little fuss as possible.