Usually they can be used interchangeably. For instance, "I went to the party also." = "I went to the party too." When you mean something was excessive though, you can't use "also". So, "It is too cold outside right now." is correct when you mean "excessively cold". "It is also cold outside right now." is grammatically correct but it means something completely different. It means both the inside and the outside (of a house perhaps) are cold. :)

You used "too" in the end the sentence, and used "also" in start the sentence.

They both mean "in addition", but their placements vary, and with them, what they apply to. One obvious difference is that only "also" can start a sentence: "Bob ate an apple. Also, he ate an orange." (But it would mean the same and be more natural here to say "He also ate an orange".) "Too" usually goes to the end of the sentence: "He ate an orange too". It doesn't have to, and one notable exception is when you want it to apply to the subject rather than the object: "I too ate an orange". "I also ate an orange" is different -- the "also" still applies to the orange.