How do you forecast quarterly growth in % for your first year?

Working on a sales forecast for my first e-commerce store (selling clothing mainly).
Since we're new I have been trying to find some competitor data which can guide us when projecting growth in percentage for the first year. This information is quite hard to get by so I am wondering how I could forecast sales growth for each Q? I assume I should be able to project how many sold items I would have in the first Q and should also be able to forecast a certain percentage growth in Q2, Q3 and Q4? But what should I base this initial assumption on and subsequent percentage growth? I would really appreciate some insight from anyone who has experience 🙂

Thanks a bunch,

A newbie 🙂