In German "let's" or "let us" translates to a type of imperative (first person plural imperative):
In some context also: lasset uns
Let's visit uncle Tom.
Lasst uns Onkel Tom besuchen.
Lasset uns beten.
But German also has other constructions to express the same meaning:
Kommt, wir besuchen Onkel Tom.
Kommt, wir beten.
Or as a question:
Wollen wir Onkel Tom besuchen?
Wollen wir beten?
Sometimes you just use the first person plural imperative of the sentence's main verb:
Besuchen wir Onkel Tom!
There are even more constructions. So, when you translate from English to German you always can use "lasst uns". It is correct German and will be understood everywhere.
But because there are alternatives, native speakers do not use "lasst uns" as often as you might expect. How much those alternatives are used depends on the geographic region. "Lasst uns" is rarely used in the southern regions (Austria, Bavaria), but more frequent in the north. This was topic of this question (asked and answered in German language):
Regionale Verbreitung von »Lass uns …«