I suppose that this one is going out of fashion, since I'm seeing more and more 'txtspk' online these days, with people simply replacing any instance of the words 'to' or 'too' with the number 2... but it still annoys me anyway!
"Too" can either mean 'in addition' or 'to an excessive degree'. For example, "I'm going there too" (i.e. "I am also going there"), or "I've had too much food", or "You're standing too close to the fire".
For pretty much everything else, you use "to", which is a word that usually expresses motion towards something (e.g. "I am going to work", "The plant has grown to six feet", "Give it to me", etc.). It has many other meanings, too - which is probably why it is occasionally confused.
Back when I first learned this rule, I remembered it as there being "too many Os in 'too'". This helped me remember only to use the word when I wanted to express that there was an excess or something.