ましょう is a conjugation of the auxiliary verb ます that is used to politely suggest doing an activity together. It is most frequently translated as 'let's (A)', or 'shall we (A)?'. To create this form, you simply need to replace the ます ending of a verb with ましょう.

• 昼(ひる)ごはんを食(た)べましょう！
  Let's eat lunch!
• ちょっと休(やす)みましょう。
  Shall we take a little break? (Said with a rising intonation to form a question)

Although it is often translated as an invitation, ましょう doesn't always target another person. It can also be a personal declaration of a strong intention to do something. This reflects the core function of ましょう as the polite volitional form (showing a will or desire to perform an action).

• 毎日(まいにち)日本語(にほんご)を勉強(べんきょう)しましょう！
  I will study Japanese every day! (I shall do it!)

In this example, the speaker is making a firm resolution to themself, expressing their commitment to making something happen (similar to the meaning of 'I shall' in English).