Hi, Adriana. Welcome to Knitting Help. PM means place marker. Typically it is something - a ring, a safety pin, a loop of waste yarn - placed on the needle to mark where you need to do something. Your pattern may also have SM, which means slip marker or move it to the right needle. The glossary tab at the top of the page has most common and some less common abbreviations and what they mean, some with videos, and it comes in handy. Be sure to ask questions if you're not sure what the pattern means. It would probably be best to start a new thread with your own questions.
You can't knit the left sock first. Ever.