Also make sure that your gamemode is not set to adventure mode.
for regular survival mode enter : /gamemode 0
for creative mode enter : /gamemode 1
for adventure mode enter: /gamemode 2
Survival mode is just the minecraft as everybody knows it and this is probably the mode you want to be changing in.
Creative is the mode in which you have unlimited blocks and resources to build everything in your imagination. You also have the ability to fly.
Adventure mode is the same as survival only you dont have the ability to destroy any blocks.
Another possibility is of course that you are to close to the spawn.
Also clicking does not break (many) blocks, you need to click and hold. Initially only a few block types can be feasibly broken using only your hand, including dirt/grass, sand, wood and leaves, so try left-clicking and holding and see if you can break these types of blocks.