I've been to several funerals, including one for a person I barely knew. Unless the family request otherwise wear either black or dark clothes. Young people can get away with a dark coat over something lighter. The family normally sit in the front rows, you should ask if should sit with your girlfriend or further back. Other people will often wait for the family to go into the church or crematoria chapel first and always wait for the family to exit before they leave the church. At some funerals people then look at any flowers that have been sent, these may be taken home by the family , left on a grave or donated to hospitals/nursing homes. However families often request only family flowers. It is expected that you express your condolences to at least one member of the family and if you have a good memory of the deceased share it - it eases grief a litte to remember good times. Turn your phone off or onto silent before you go into the ceremony.
If hymns are sung and you dont know them mime
After a funeral it was traditional to go back to the house for a snack and a drink but only if invited and acquaintances can make a polite excuse not to go. .This is again a time for fond memories of the deceased and even for funny stories as long as they arent obscene. You could ask if they'd like you to hand things around or make tea, but dont feel obliged to do so. .
Your girlfriend will probably cry in the church so do take tissues but dont be surprised if it turns into something that is almost a party at the house.
What should I do?