Does your bike not want to shift? Is it making weird clicking noises? Does it shift slowly? Chances are your bike needs some indexing. Sometimes cables can stretch throwing off the shifting. Or it could be the gnomes were messing with the adjustments on your bike.
Before you start, think about when your shifting started working bad. If it has just been a slow change indexing will probably fix your problem. But if it went bad after some sort of incident like a crash, you might have another issue to fix first. Crashes and part failures can cause a sudden issue with your shifting.
For example, I recently had a chain with a break in one of the links. The chain still worked, but it really messed up my shifting. The bike was ghost shifting a lot, this is when it changes gears without a shift. I didn’t realize the chain had a break till I started trying to index the shifting. Another common problem is a bent derailleur hanger. The hanger is designed to bend or break in a crash, so your frame isn’t damaged. So, if you’ve recently hit the derailleur on something that could be the problem. But if the chain looks good, you haven’t had any recent crashes, and the shifters seem to function correctly, it is probably the indexing that needs adjustment.
Tools needed: screwdriver for limit screws, bike stand, maybe a needle-nose pliers. This is a great screwdriver you can buy at Amazon for limit screws:
Here is a great video from Park Tool on indexing your rear derailleur. I strongly advise you follow the beginning of the video and get your limit screws correct. The first few times I tried to index I figured my screws were already set and went right by that section. Well, the limit screws ended up being my problem. They are your starting point for indexing so if they are wrong you won’t get great shifting. Park Tool has a lot of great videos on youtube for bike repairs. When you can, I recommend paying them back by buying some of their tools and other products.
If you have a front derailleur the indexing is similar, here is a great video for the front derailleur.
There is of course worn parts that could be your problem, but this should fix the majority of shift problems.