Selling Online / Etsy

I started a side job selling clothing on Etsy and through social media. I'm tracking my sales and expenses but was wondering if I was going about it correctly. I'm tracking all the inventory I buy each month along with my sales for the month. I often buy more inventory than I sell. So am I working at a loss for that month? And should I be setting aside a portion for taxes? If so, how much? 10-30%?