Try to have a baby shower if you can! And ask around to friends/coworkers/family. And maybe call your counties district building and ask about programs. You can get started on WIC once you find out you're pregnant and that helps with food and formula once the baby is born if you're not breastfeeding. They also may have numbers or information on programs to help! Thrift shops are also cheap like the above poster said, but definitely don't rely on "free stuff" I was 18 when I found out I was pregnant and had just quit my job so I had to find a new job so I could save money and help provide all the essentials for my son. Unfortunately you won't get everything for free, but you will and can receive help!
Look into programs in your area that help young moms out. Thrift shops are always cheap too