There are a few ways to start expressing your opinion in Lithuanian. The following are the most popular phrases to do that:

aš manau, kad... I think that...
aš galvoju, kad... I think that...
man atrodo, (kad) ... it seems to me that...
mano nuomone, ... in my opinion, ...
mano galva, ... in my opinion, ...

Aš manau, kad jau reikia eiti. I think that it's we need to go already.
Aš galvoju, kad jis važiuoja namo. I think that he is driving home.
Man atrodo, kad ši mašina yra žalia. It seems to me that this car is green.
Mano nuomone, tu esi blogas mokinys. In my opinion, you are a bad pupil.
Mano galva, šis autobusas važiuoja lėtai. In my opinion, this bus is going slowly.

You can also change the persons of these phrases:

Tu manai, kad man reikia eiti. You think that I need to go.
Jis galvoja, kad yra gražus. He thinks that he is pretty.
Mums atrodo, kad jūsų mašina yra žalia. It seems to us that your car is green.
Vytauto nuomone, aš esu bloga mokinė. According to Vytautas, I am a bad student.
Tavo galva, jis jai patinka. In your opinion, she likes him.

The new verbs that are used in the phrases are:

manyti (jis mano) 3rd type - to think
galvoti (jis galvoja) 1st type - to reason; to think
atrodyti (jis atrodo) 3rd type - to seem

And two more nouns:

nuomonė - opinion (In the phrase "mano nuomone", nuomone is in the instrumental case.)
galva - head (In the phrase "mano galva", galva is in the instrumental case too.)