Why was Light a suspect in the first place?

I haven’t watched Death Note in years, probably due for a rewatch. But for some reason I was thinking of it. Why was Light ever a suspect in the first place? What did he do to make L suspicious? He was just a high school kid. All he had to do was hide in his room alone and write the names. It seems impossible to screw up. If anyone can jog my memory that’d be awesome. Thanks