How to I make the bet that Deutsche Bank price will go down to $5 in the next 6 months? Sell short or put?

Obviously I’m not an expert at this, but I want to put some money into this play…likely no more than $500

Looks like the price of a put for a $5 strike expiring 10/20 is about $.45.

A) Do I want buy to open, buy to close, sell to open, or sell to close to do?

B) Schwab says I don’t have enough money in my margin account even though I have $700 in there…so how much do I need in the account?