Learn Farsi: لطفاً اینجا امضا کنید.

The verb 'to sign' is a compound verb: emza kardan (literally 'to do signature'). When making commands with compound verbs that use kardan, Farsi speakers usually drop the be- prefix. So, instead of emza bekonid, it is simply emza konid.

How can I tell if this command is formal or plural?

The verb ends with -id (konid), which is the ending for the 'you' plural or formal pronoun (shoma). You would use this sentence to address a group of people, or to be polite to a single person like a customer or a colleague.

What is the little double line at the end of lotfan (لطفاً)?

That symbol is an Arabic marker called a 'tanwin'. It adds an '-an' sound to the end of the word, turning the noun lotf (favor or kindness) into the adverb lotfan (kindly, or please). In Farsi, you will usually see it sitting on top of the letter alef.

Is inja one word or two?

It is used as one word here, but it is built from two basic Farsi words: in (this) and ja (place). Together, inja literally means 'this place', which is how you say 'here'.