We make beer and the bottles often end up left for weeks after drinking, resulting in some pretty hard-to-remove dried yeast residues near the bottom. That stuff is hard to remove. Warm water with dish soap, leave it to soak for a bit, empty it, stick some sharp sand in it along with a bit of water and shake it thoroughly.
If you want it fully sterile, you'll need to use a sterilising solution or -like we do- boil the bottle for a few minutes.
Careful not to put a cold bottle in boiling water, that will probably crack it in no time. Pour some warm water in it first and let it get to temp, then into the boiling water.