The use of "for me" and "to me" really depends on your meaning, and there is no single definition. 

Examples:

John is writing an email for me --> he is doing something on your behalf. 

John is writing an email to me --> John will send the email to me.

This pair of jeans is too small for me. --> I am specifying the suitability of something for me.

Leave some food for me, I'm starving! --> Asking someone for a favour.

To me, this is simply unacceptable. --> I am stating my opinion or belief on something.

It is important for me to dress well to a wedding. --> My opinion based on societal expectations.

No problem, leave it to me. --> I am offering to take over someone's duty or responsibility.

That parcel is for me, bring it to me. --> I am the recipient of the parcel, and someone will bring it towards me.

Your sentence: It's meaningless for me or to me?

Some context is missing as we do not know what came before your sentence. If you are indeed stating your opinion of something, then "to me" is more suitable.

Hope this helps.

Lance

Using "to" or "for" depends on the phrase which comes before it. The easiest way to learn this is to just memorise the expressions.

We don't know your context, but "meaningless to me" is probably what you want.

You can do a search of each phrase to see which is correct.