Start with things that don't have sentimental value, such as knick-knacks and things bought on impulse,
Move on to things you have not used for years and see no forseeable use for in the future.
Harden you hearts and do not allow the thought "it might come in useful one day" to cross your mind. It HASN'T anytime these last twenty years, has it? No, so OUT IT GOES!
Photos? Ask yourself two questions, are they clearly labelled as to who these people are and what the occasion was, and will they mean anything to those who come after you?
I inherited a vast number of family photos of people w hom I have no idea who were, only which side of the family they probably belonged to. No use, at all. The photos that actually mean something to me, I have kept - the rest I disposed of.
Or you can simply put them in a large box labelled old photos and leave them for the next generation to throw out.
Alteratively, you can scan them all in and store them on an external hard-drive. This takes time, but saves a lot of space - however, you will need to remember to upgrade as computer programs change!
Books, records, cds, tapes - are you going to re-read them, listen to them and do you have a record player, tape record or cd player you can use? If not, there is little point in keeping them.