While I am by no means an expert, as a serious cyclist there are some things I have noticed that I would like to share with my readers, some of whom are thinking about taking up cycling in a more serious manner.
1. Almost everyone has their bicycle seats set too low. Even when I got my first good bike, the bike shop set it too low because they simply asked me what felt right rather than suggesting setting it at the proper height.
For most efficient pedalling a bike should be set so that when your pedal is at the very bottom of the rotation there should just be a very slight bend in your knee. An exception is for mountain biking where in the case of downhill or very technical trail riding you might set it with a bit more bend. I notice most casual riders have an extreme amount of bend in their knees when they are cycling. If that is you, try reducing the bend and you will notice the difference.
2. Almost everyone keeps their tire pressure too low and rarely checks it till it is obviously almost flat. If you are always riding on pavement you should keep your tire pressure near the recommended maximum stated on the tire. This will provide more efficient cycling with less rolling resistance and less effort required.
My hybrid tires' maximum is 75 PSI but I run them at about 60 because I ride my hybrid on gravel and easy dirt trails as well as pavement. For that reason I also use hybrid tires with a fair amount of tread on them.
My mountain bike tires', used for trail riding, maximum is 50 PSI but I run them at about 40, which is fairly normal, though some mountain bikers run much lower tire pressures in certain conditions. Unless you know why you are running really low tire pressures avoid them.
Hard core roadies with serious road bikes and extremely skinny smooth tires will run at very high pressures, but if that is not you it is probably best to keep to under 80 PSI and never more than the maximum rating on your tires.
3. The majority of cyclists neglect to keep their drive-train (chain and gears) lubricated. Preferably one will check and re-lube their chain regularly with special bike lube. But if you are a casual cyclist with an inexpensive bicycle you are probably better just using heavy grease that only needs to be reapplied a few times a season. That way you will be sure not to have your chain running dry.
I ride an inexpensive bicycle in the winter to avoid exposing my good bikes to salt damage and I use lithium grease on it, supplemented with chain saw bar oil to deal with the slush and salt on the roads in the winter.
I also want to add two further comments that should not really need mentioning but by watching a lot of cyclists out there they obviously do need to be emphasized, and that is headgear and footwear.
Always wear a helmet, You never know when you will need it. It is not a panacea but it definitely can be the difference between walking away with no harm or with a head injury, in particular a concussion. And in worse case scenarios in can mean the difference between a serious injury and death. Being alive is far more important than looking "chic".
And please, no flip-flops or sandals. Never wear footwear that exposes your feet or toes. Decent running or walking shoes are fine. In the winter I wear my winter hiking boots. You don't even want to think about the pain of your exposed toes being dragged along pavement or gravel.
And finally, I will mention two other things I have noticed. Parents seem to think their children are smarter than them and thus protect their children's brains but not their own, essentially sending the message that "once your older you won't have to wear your stupid helmet". And teenage girls seem to think that keeping a spare helmet hanging from their handlebars is more important than having one on their head. Perhaps they think being pretty is more important than being smart, but trust me, a smashed brain is not a pretty sight.
These are not the only things one should know about cycling but they are the most obvious things I have noticed novice riders neglecting.