There are 45 categories of classes for the definition of goods and services. Most Trademark Offices are using the nice classification for classifying goods and services. The classes are defining the area of business for which you want to register your trademark(s).
You can search and identify your classes via tmclass.tmdn.org however classification is a very important point of your registration, we highly advice that you look for help to register your Trademarks and define the right classes. Having defined the right classes can accelerate the procedure of registration.
EUIPO advises to think big and they are right. You might have more goods and services than you think, go through all of your services and check for cross services or products. Also think where you want your business to be in the future, what kind of services and/or products would you like to offer in the future. Don’t forget your trademark will be registered for ten years.
Be realistic, is another advice EUIPO is proposing, in choosing where you want to go in the future, because if you chose classes for services or goods that you are no using will make you vulnerable.