I want to say:
First, let's see what is the concept of (a word).
The question is how can I write this part of the sentence (let's see) in a formal manner?
I want to say:
First, let's see what is the concept of (a word).
The question is how can I write this part of the sentence (let's see) in a formal manner?
There really is no clear definition of 'formal' speech, except perhaps to avoid informal, or slang terms. There is nothing particularly informal about the expression "let's see".
Two things you could consider to make it more formal:
Contractions are not strictly informal, but they mostly come about through everyday speech and so removing them arguably adds some formality.
These suggestions may give an impression of more formal speech, as there is a widely-held concept that formal language is more verbose, and that educated or well-read people have wider vocabularies and would choose lesser-used words and expressions. But really, any speech that is clear and as free from slang as possible could be acceptable in a formal setting.