When you use the chunk "Let's", you must add a verb that expresses your suggestion. This form of the verb must be the infinitive form, no matter how many people you address:
Let´s have a look at Unit 7 " Animals".This topic is really interesting and there are many things to learn ,so we can start by dividing animals into 2 important groups:
Vertebrates and Invertebrates
Play the movie and then have a go with the game
We are going to learn the Animal classification ,too. Click and find out!