Respect and etiquette play a significant role in French culture. When tourists complain that the French are rude, they might have had such an experience because they didn't follow the norms of respect and politeness during their visit. But if you try hard to show respect, you will be treated with respect in return--and people will even go out of their way to be kind to you. Here are some basic rules to follow to make sure you outwardly display the respect you feel towards others in France.
Step 1: Titles of Respect
In France, it's important to address everyone--yes, even young people--with "Monsieur" (for a man) or "Madame" (for a woman). The French call people by the French equivalents of "sir" and "ma'am" much more often than Americans (except maybe Southerners--in France, just pretend you're a very well-brought-up Southern young person who's constantly saying "sir" and "ma'am" and you'll do perfectly). Even when you're just stepping into a small shop to look around or you pass your neighbor in the stairs or hallway in your apartment building, make sure to say "Bonjour, Monsieur" or "Bonjour, Madame." This shows that you're acknowledging the other person's presence and you respect them enough to give them a courtesy of a simple "hello."
Step 2: Dressing Respectfully
The French are very much into aesthetics. They appreciate beauty and want to keep their city looking its best. This includes not only the permanent fixtures, like buildings and landscaping, but also the temporary fixtures: people. When Parisians leave the house, you'll almost never catch them wearing athletic clothes, leggings, sweatshirts, etc. (yes, even if they're jogging--I've stopped doing double takes when I see joggers wearing sweaters and dress pants because it's the norm, not the exception). Both men and women take care in the way they dress and groom themselves to make sure they always look put-together. This way, they won't mar the beauty of the city by looking sloppy and uncultured, and they let others know they respect them enough to put in the effort to look decent when they're going to be seen. When you leave the house in Paris, try to avoid wearing shorts, sweatpants and sweatshirts, or leggings. As far as grooming, at a minimum, brush your hair. Pretend you're going to see your grandma for a nice Sunday lunch--even if you're just going across the street to get a kebab.
Step 3: Saying "Vous"
If you're talking to a stranger or someone older than you or in a position of authority (like a teacher, doctor, police officer, school administrator, etc.), make sure to use the formal pronoun "vous" instead of the informal "tu" when you're talking to them. To "tutoyer" (use "tu" with) someone before it's acceptable to do so (like if a professor specifically tells you it's okay) is seen as very disrespectful. Basically, if you'd address the person with "Monsieur" or "Madame" (so they're not your friend or someone at "your level," like a fellow classmate), you need to "vouvoyer" (use "vous" with) that person.
Step 4: La Bise
When friends (and even friendly acquaintances) reunite, even if they recently saw each other, they give each other kisses on the cheek--not real kisses, but they put their faces next to each other and do an air-kiss on each side (left side, then right side). Even men do this and it's not seen as girly or feminine at all. If someone does "la bise" with you, it's polite to go along with it, and instead of being freaked out you should really take it as a nice sign that this person feels fairly close with you. Also, when French people go to a social gathering of friends or family, they go around to greet everyone and "faire la bise" before settling in one spot and getting into a conversation or getting something to eat or drink--so don't expect to get into a long conversation with your French friend right away if he or she just walked in the door.
Step 5: Eye Contact
In the street and on the metro, you only really make eye contact with people if you want to talk to them. If you're staring right into someone's eyes in the metro, especially if you're smiling, this can be taken as a sign that you want to talk to that person and they might think you're romantically interested. Like in most big cities, Parisians are in their personal little "bubbles" and don't go around smiling and making eye contact with strangers all day as they walk around the city. If you want to be left alone, leave other people alone and don't get into their "space" by doing those things.
Step 6: Saying Excuse Me
When you have to closely pass someone in the street, when someone has to pass you, or when someone holds a door open for you, say "Pardon," which is the French equivalent of "Excuse me" ("excusez-moi" is when you're about to start talking to someone, like to ask for directions). Otherwise, you'll probably be seen as rude and/or arrogant. No one wants someone nearly knocking them over as they brush by and then not even acknowledging it, so make sure to let the other person know you're sorry for inconveniencing them and getting into their personal space.
Step 7: Perception of Time
The French value the enjoyment of life and don't see business matters and making money as the most important things in life. This means that shops, restaurants, and offices often close for a few hours in the afternoon to take lunch breaks and have a relaxing pause in the middle of the day. They don't appreciate if you arrive so close to closing time or break time that there's no way you'll be gone by their official closing hour.
In terms of being on time, the French don't stress out about it as much as people in most of the US. For example, if you're invited over to someone's house or to anything your friend or acquaintance has to prepare for, it's respectful to actually show up just a bit late so you don't make them feel rushed to get ready. However, to show you respect the time of the employees and other customers, you should make an effort to be on time to things like restaurant reservations and appointments (but being a few minutes late isn't outrageously offensive).