I'm tired, so I think I'll go to bed now.

Literal

Tired [because-ので] already bed [into-に] enter let's [quotative-と] think.

もう床に入ろうと思う combines the volitional ~ろう ('let's/I'll') with ~と思う ('I think'): 'I think I'll go to bed.' A natural expression of intention prompted by tiredness.