Do you need a comma before or after "too"?

Too is an adverb. It's usually used to mean "in addition" or "also."

It is one of the hardest words to know whether you should use a comma or not. It really depends and many editors will have contradictory views on this punctuation. If you want to emphasize the "too" at the end of a sentence then use a comma, but if it's not that important then you can get away with leaving them out:

• I'll come, too.
• It had rained on every other birthday too.

I like you, too. (emphasis) and I like you too. (no emphasis) are both correct,

If "too" comes in the middle of a sentence then you should either have two commas or no commas. If you just have a single comma before or after then that's definitely wrong.

Correct:

• I, too, like being with you. (emphasis)
• I too like being with you. (no emphasis)

Incorrect:

• I too, like being with you.
• I, too like being with you.