To - too - two
Two is the number '2'.
Too (pronounced long) means
- a) as well, also. E.g. 'I like coffee. I like tea too.'
- b) overmuch, surplus, excessively. E.g. 'The weather is too hot for me.'
To is the spelling for all other meanings of words that sound like this. 'To' tends to be pronounced very short, IPA: /tʊ, tə or tə/, as in 'He's gone to (/tə/) London' or 'You need to (/tʊ/) understand her point of view'. Sometimes, in the representation of informal speech or slang, 'got to' is written as 'gotta'.
A short example of a sentence which may help you to be clear about these three words is the (rather abusive) e-mail note: 'Two fingers to XXX too'.