We have one te too many. We can delete either. I think que mais te diz is how most people would put it nowadays; so I’ll leave the que te mais diz, which has a quaint, 19th century feel to me, in keep with the rest of the sentence, I think:
Decora o que te mais diz, meu bem, e logo te darei mais que decorares...
This translates as:
Memorise that which you most relate to, my love, and then I’ll give you more to memorise.
Now the o que you stumbled on. You on can translate it simply as what, as in:
Faz o que quiseres (do what you want)
Come o que mais gostares (eat what you like best)
Diz o que tens a dizer (say what you have to say)
But we can look at the two words. The que is the relative pronoun equivalent to that, which, or sometimes to who, as in:
A carta que chegou hoje (the letter that arrived today)
O carro que eu vi (the car that I saw)
The o is here a demonstrative pronoun (Aulete 4) equivalent to aquilo; so that. So the whole thing is memorise that that means most to you. I wrote that which to avoid repeating that. Or you could say memorise what means most to you.
In the second bit — te darei mais que decorares — is rather unusual. I would say mais que decorar, not decorares. This que, is a relative pronoun without antecedent: it does not refer to anything mentioned earlier, unlike its cousin in a carta que chegou hoje. It is equivalent to coisas que, algo que: so que decorar = coisas que decorar. This construction was more common in the past, but I’ve never seen it with the personal infinitive (as decorares); always with the straight infinitive (as decorar). Nowadays people would more commonly say mais para decorares, which more closely matches the English more to memorise.
You had interpreted this bit, mais que, as more than. Well maybe that’s what was intended, and the author didn’t get it quite right. Maybe they meant to say mais (do) que coisas que decorar.
On a final note, the verb dizer — o que te mais diz — is being used idiomatically in the sense of (Aulete 11):
Provocar interesse, atração; ter significação para (alguém). [tdi. + a, para : Esse lugar lhe diz alguma coisa?]
[my translation:] To raise interest, cause attraction; to have meaning to (someone) [ […] Does this place attract/interest you in any way? Do you relate to this place in any way?]
So, memorise the words that mean the most to you, and soon you’ll have more. Lucky you!