Just some words for the first time traveller to China:
It's a huge place, more like a continent, so take your time, and avoid those refugee style tours which cover 10 places in five days.
It's a nice thing to sit down, enjoy the airs, sights and smells, meet people, especially since we speak their lingo, and immerse yourself in the place like a local. You will enjoy it more than if you merely sat in the back of a tour bus, seeing everything from the comforts of a bus.
Some places, or rather many destinations require some form of walking, so being fit helps. You will see much more.
Food can be very different, especially for those who have only taken Canton food. Try them, be it insects, snakes etc.
China is very crowded, and no matter where you are, you will see lots of people. If you don't like that, don't go and then complain. Chinese can also be very pushy, and it will take another essay to explain why. When in Rome, do like them and you will be fine.
The toilets, especially those in rural places can be 'exotic'. If you can't stomach that, stick to the major cities and don't go somewhere that needs a pit stop along the way. If you need to go, the fields are often cleaner than the toilets. Bring some deodorant if you like.
Different seasons bring different sights, so talk to your travel partners and plan ahead, bring the right clothes.
Try to visit one county, eg, Canton or Shanghai first, and then move on to a new place. Otherwise you will find that China is a big big place and you will spend 80% of your time on a bus.
DON'T Drive in China. Even if you think you are Lewis Hamilton, the traffic conditions are bad and people will ask you for money for any old injuries.
There is very little serious crime, but petty crimes do happen. You may find some cheating, pickpockets, fake products or overcharging, but no robbery or murders. Even so, be smart, do not act like a typical Singaporean, leave things everywhere unguarded. I have seen people leave their handphones to book seats - are you kidding ???
Finally, yes, do go to the big few like the Great Wall, Nine Gorges etc, but do consider the less traveled places, and you will be rewarded with a different view of China. The people are very friendly, helpful and there are many things to see, if you know what to expect. Don't go around complaining, and criticising everything, you will make more friends. It's a good time to brush up or improve your Mandarin. Don't get mad if people don't speak English, they don't.
For less travelled places, try:
Xining - Qinghai Lake
JiLing - near N Korea
Yunnan - Li Jiang and Xi Shuang Bana, attend a minority tribe wedding