If you are dining with Japanese people they will understand that you don't know the rules. They will probably forgive you if you commit some major faux pas. However, knowing a little about the local manners really goes a long way in making friends, winning business and just generally being a good guest.
I am not going to lie to you, its not easy. Eating with chopsticks is an art and even Japanese themselves often struggle with perfecting it. However, the good news is that knowing these simple rules gets you 99% of the way to perfect politeness:
1. Hold your chopsticks correctly
Easier said then done. This is the part that really takes some time to master. Watch how other people are doing it and be patient with yourself. If you really want to learn you should get as much practice as possible. Eat with chopsticks at home and don't get lazy holding them just because no one is watching. You will find that when you hold them properly they are much easier to use anyway.
2. Don't eat directly from common dishes
Take food from the shared dishes and place it on your own plate or bowl before eating it.
3. Use your chopstick holder
Many Japanese restaurants will provide a chopstick holder. When you are not using your chopsticks place them on the holder. If the chopsticks are disposable you will not be given a holder. However, you can make one from the wrapper of the chopsticks. Chopsticks should never be placed upright in your rice as this resembles a ceremony performed at funerals in Japan.
4. Don't browse with your chopsticks
Don't hover your chopsticks over all the dishes when thinking what you want. This is considered greedy (sashi bashi).
5. Don't dig
Take food from the top of the dish. Don't dig in the dish looking for something good :).
6. Don't lick
Don't lick the ends of chopsticks. (Neburi bashi)
7. Be careful giving food to others
Never share food by passing from chopsticks to chopsticks because this resembles a custom at Japanese funerals when cremated bones are ceremoniously transfered to the urn. This is probably the biggest taboo at the Japanese dinner table. You can transfer food using your chopsticks to someone else's plate but get them to pass the plate to you if it is a distance. Ideally you would ask the server for another pair of chopsticks that are placed in the center of the table and used whenever someone needs to transfer food for another person.
8. Chopsticks are not a toy
Don't point with your chopsticks when talking or hold them for extended periods of time without eating. Never rub the chopsticks together repeatedly after you break them apart because it is a sign that you think the chopsticks are cheap.
9. Don't cross your chopsticks when resting them on the table
Again, your chopsticks belong in their holder and make sure they are parallel to each other when resting. Crossed chopsticks are another thing that remind people of funeral ceremonies.
10. Don't swirl your chopsticks in your soup
When you do this it looks like you are trying to clean them. This is sometimes tempting because miso soup remains suspended and does not dissolve. Resist the temptation!
In general the most important rules are the ones that remind people of Japanese funeral rights. Remember that chopsticks are not just two sticks for eating but are deeply ingrained culture symbols. Chopstick manners differ from culture to culture and in China and Korea the rules are different.
If you try your best everything will go well, don't get too uptight about the rules and with controlling your chopsticks. Chopsticks are challenging for everyone and even Japanese people rarely have mastered the art.