Goals for velo are tricky, there is the "gold standard" of 3-5MPH per year
But there's also different contributing factors such as puberty, over all strength, physical maturity are you a late bloomer, how clean your mechanics are, etc.
As for how that 3-5 potentially could develop over the year, a rough guideline to follow could be something to the effect of:
First six weeks: Immediate jump in velocity
Second six weeks: No gain in velocity
Next eight weeks: Very slight gain in velocity
Sometime after week 20: potentially another big jump in velocity
Hope this helps