We can calculate the concentration of H+ ions in a 0.1 M solution of acetic acid by using the pKa or ionization constant which for any compound is the negative logarithm of the equilibrium coefficient of the neutral and charged forms.
pKa = -log(10)[Ka]
Ka for acetic acid is [H+][CH3COO-]/[CH3COOH] where  indicates the concentration.
pKa = log(10)[CH3COOH] - log(10)[H+] - log(10)[CH3COO-]
as [H+] = [CH3COO-]
pKa = log(10)[CH3COOH] - 2*log(10)[H+]
For acetic acid pKa = 4.76 and -log(10)[H+] = pH
4.76 = log(10)(0.1) + 2*pH
=> 2*pH = 4.76 + 1
=> pH = 5.76/2
=> pH = 2.88
The required pH of 0.1 M solution of acetic acid is 2.88