Print off your past 3-4 months of bank statements and do the actual math on what your income goes to (it may be a little painful, but it is incredibly enlightening) use that data to see what you are spending on essential items and how much is going to splurging, and create a budget *on paper*. There are some great word/excel templates online for this, there are apps as well, but I always encourage people to have a plan articulated in writing for their finances.
Also, make a LIST of the things you are planning to buy and don't deviate from it.
I like to keep a running list in the margins of my organizer of all my nonessential upcoming purchases and that way when payday rolls around I pick what I buy from my list and what can wait a little longer (mine currently consists of hair conditioner, travel tickets, dog nail trim, black flats, getting a map framed, tea cups, clothing alterations)
Also, whatever you own that you don't use...sell it or donate it.
A a 300-400 surplus a month can add up very quickly to a few grand if you are diligent, and you can start setting goals for yourself for an emergency fund, travel expenses, future rent, investing, etc. If you're living with your parents, this should be the time that you get super aggressive with your savings, because your expenses will probably never be any lower than the present.