La crema solar

Question

Is it necessary to use definite article? What difference would it make if we dropped la?

Answer

Hola Emanuel
In this sentence:

Cuando os tumbéis en la playa poneos la crema solar.
When you lie down on the beach put some suncream on.

you can drop the article if you want. If you do, then the speaker is thinking about "crema solar" in a genaral way as in "any suncream".  Using the article means that the speaker is probably thinking about a specific suncream, i.e. a bottle of suncream that he/she knows they have  in their bag, for example. 
Another example could be:

1. Pon cucharas en la mesa para comer el postre. (general-any spoons)
2. Pon las cucharas en la mesa para comer el postre. (specific- thinking about the spoons they have in the drawer, for example)

I hope this helps,
Inma