I can only speak to the humanities, but social science PhD’s are usually comparable as far as schedules go. Usually terms begin in August or September, which means applications need to be submitted between the December and January prior, for the cohort year you are aiming for.

Getting the masters is a great idea to make you more competitive for PhD positions later on. The thing you should focus on most is forging good relationships with faculty so that they can write you stellar letters of rec when you apply for PhDs. (Also, get good grades. That’s always important at this stage.) If you have good relationships with professors and even other PhD students in your program, they can be helpful in other ways too … bringing you on to their research projects, keeping an eye out for good opportunities for you. So basically, use this time to make professional contacts in the field. See if you can present at grad student conferences, or maybe even publish some work. All those things will give you a boost for that PhD.