Learn Spanish (Latin America): Ellos van a acampar cerca del lago.

To say you are "going to do" something in Spanish, you always use a form of the verb ir (to go), followed by the word a, followed by an infinitive (an unconjugated action verb). Here, van (they are going) + a + acampar (to camp) creates the near future meaning. You cannot skip the a!

Where does the word del come from?

del is a mandatory contraction. When the preposition de (of/from) comes right before the masculine word el (the), they squish together to form del. So, de + el lago becomes del lago. Note that this only happens with el, never with la, los, or las.

Why do we need de (inside del) here? Doesn't cerca already mean 'near'?

In English, we just say "near the lake", but in Spanish, the phrase is cerca de. When you state what something is near, you must use de to connect cerca to that place. Since "the lake" is el lago, cerca de + el lago turns into cerca del lago.

Is the word Ellos mandatory at the beginning of the sentence?