First of, you'll need a start and stop codon (let's use UAG for start and UGA for stop).
The first sequence we need to make is the DNA sequence. The start and stop codons only work for the mRNA strand because every base on mRNA is transcripted from DNA so the bases won't be the same. So put the ANTIcodons of the start and stop codons (opposite of the start and stop codons) at the start and end of your sequence correspondingly.
ATC --------- ACT
You already have 6 bases so just subtract 6 from 15 (which is 9) to calculate how many bases you need in between the start and stop codons. Now just make up a random sequence.
e.g ATCAGUCGAUGUACT
So that's your DNA sequence.
Now we can transcript this sequence to make the mRNA copy (here's where the start and stop codons show up). Essentially just do the opposite bases (make sure to swap every Thymine base with a Uracil base as it's RNA not DNA):
UAGUCAGCUACAUGA
This is your mRNA copy sequence.
To translate this, you will need a table of amino acids corresponding to all their possible codons. Find one on Google or something and use it to translate this sequence into the primary structure for the protein.
