Who can declare war on another country?

Anyone can declare war on Another Country. That does not mean all will follow you, nor make it 'legal', if that has any meaning here. It is of course also possible to declare war on your own country, the tragic results of which are there for all to read.
For a country to declaring war on another country is a different matter, it would take the ruling government to decide to go to war. There may be provisions for it within documents or simply the authority of the ruling party or person.
Usually only the President, Prime Minister or Cabinet of a country can declare war