Learn Spanish (Spain): ¡Mira esto!

This is an affirmative command, used when telling someone to do something. To give a command to one person informally (tú), you use the same form as the 'he/she/it' present tense form. For the regular verb mirar, you drop the 's' from miras to get mira.

How would I say this if I were talking to a group of friends?

In Spain, when giving an informal command to a group (vosotros), you simply replace the final -r of the infinitive verb with a -d. So, mirar becomes mirad, making the sentence ¡Mirad esto!

Why is there no Spanish word for 'at' in this sentence?

In English, we have to say 'look at' something. In Spanish, the verb mirar already means 'to look at' or 'to watch'. You just put the thing you are looking at right after the verb, without adding any extra prepositions.

Why does this use esto instead of este or esta?

Esto is the neuter form of 'this'. You use it when pointing out a general object, idea, or situation and you haven't named exactly what it is, so you don't know its grammatical gender yet. If you were pointing specifically at a masculine noun like a book (un libro), you would use este instead.