You can translate “Let’s go.” into “Iko”/「行こう」in Japanese and you can say it when you invite someone to go somewhere together. There are other expressions and they have subtle differences. I will introduce them.
This is a casual expression, so you can say it to your friends and family. I think more commonly Japanese people say “Ikoyo”/「行こうよ」or “Ikoka”/「行こうか」. “Ikoyo”/「行こうよ」includes the nuance of “Why can’t you go ?” or “Would you like to go earlier ?”.
When it’s the time when you have to leave for the restaurant you already reserved
A:行こう or 行こうか
When your friend is reading a book although it’s the time when you have to leave for a station
This word is a polite expression, so Japanese people say it to elderly people or someone you aren’t close with. You can also say “Ikimasyoyo”/「行きましょうよ」or “Ikimasyoka”/「行きましょうか」. When you say “Ikimasyoyo”/「行きましょうよ」, you can give people the nuance of “Why can’t you go ?” or “Would you like to go earlier ?” same as “Iko”「行こう」.
This is also a casual expression and it can give people a little compulsory impression. I have a stereotypical thought that normally women don’t use it but some men use it.
We also have “Ikuka”/「行くか」which doesn’t have a compulsory impression. I sometimes talk to myself as the following example.
When it’s the time when I have to leave for the workplace
A:行くか (talk to myself)
This expression is a very casual expression, so you should try not to say it to elderly people or someone whom you don’t know because they can feel you are too friendly. I often say it to my daughter who is 4 years old.
“Ikokka”/「行こっか」has a very casual impression. “Iko”/「行こう」is a casual expression. “Ikuzo”/「行くぞ」is a casual expression but also has a little compulsory impression. “Ikimasyo”/「行きましょう」has a polite impression.