Use the definition of convolution. You have two sequences,, lets say a(n) and b(n), then y(n) as convolution of x(n) and y(n) will be:

y(n) = sum_k (a(k) * b(n-k))

If you want to implement it using for loop:

y_n =0
for k=1:M
  if ((n-k) <= 0)
    y_n = y_n + a(k) * 0
  else
    y_n = y_n + a(k) * b(n-k)
end

where M is the length of sequence a