I've just changed a worn out cassette on my road bike (SRAM 8 speed 12-26T) and replaced it with a Shimano 12-25T 8 speed cassette. Now it looks like my gear shifting doesn't work. I can't get into the lowest gear, and there's a clicking sound when I try. I know almost nothing about derailleurs. What is my best option now?
As I stated in my comment,you need to rearrange your limit settings after switching the cassette, you can have a look at this video, if the limits are not set correctly the chain could either fall off the cassette or not being able to reach the smaller/bigger cog.