Im trying to figure out the best way to buy tickets for metro north?

i just move to greenwich ct from nyc 2 weeks ago and i work in the city from thursday tru monday im trying to figure out the best way to buy tickets for the metro north i want to get monthly pes but i been reading online they are only good for the weekdays i have to get another type of ticket for the weekend im a little confuse i mean a whole lot confuse since im new to this can some one please help