The first thing to do is to start building your tree here on Wikitree, entering the information that you have - you have some dates, you have a place, you probably have a spouse and children. Then others can help in your search or you may find that there are other family members already here that you can connect to.
Is this your Peter in the 1920 census? If so, this tells you what year he immigrated so you can look for immigration documents. The 1940 census says he was a naturalized citizen; citizenship papers give all sorts of information about birth and parents.
Name: Peter Muller
Age: 39
Birth Year: abt 1881
Birthplace: Germany
Home in 1920: Union Ward 1, Hudson, New Jersey
Street: Palisade Avenue
House Number: 135
Residence Date: 1920
Race: White
Gender: Male
Immigration Year: 1902
Relation to Head of House: Head
Marital status: Married
Spouse's name: Mary Muller
Father's Birthplace: Germany
Mother's Birthplace: Germany
Native Tongue: German
Able to Speak English: Yes
Occupation: Carpet layer
Industry: Linoleum
Employment Field: Wage or Salary
Home Owned or Rented: Rented
Naturalization Status: Papers Submitted
Able to Read: Yes
Able to Write: Yes
Neighbors: View others on page
Household Members:
Name Age
Peter Muller 39
Mary Muller 41
William Muller 14
Frederick Muller12
Emil Muller 9
Edward Muller 5
Christina Muller3
Francis Muller 0
You can also find him in the 1910, 1930, and 1940 censuses. Here's a link to the 1920 census on FamilySearch which is a free site. "United States Census, 1920," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M4T3-GSV : accessed 4 August 2020), Peter Muller, Union Ward 1, Hudson, New Jersey, United States; citing ED 296, sheet 1A, line 1, family 1, NARA microfilm publication T625 (Washington D.C.: National Archives and Records Administration, 1992), roll 1049; FHL microfilm 1,821,04