The first recommendation I would make is to really understand Open To Buy. How to calculate it, how to update the OTB before making noticeable purchases, and how to use it to understand the margins from various types of purchases.
Secondly, I believe it is important to understand merchandise planning in general. Basics, like socks and underwear, are best managed using a stock level budget. You establish your inventory levels by product class, then by style, color and size. Buy in your initial inventory, then monitor it and adjust the base levels up or down at least monthly. Orders for this class of merchandise should not be controlled by OTB as you are simply replenishing basics you have sold.
Fashion basics, like navy blazers also need to be on a replenishment program for year round products.
Where a lot of retailers fail is controlling their OTB in fashion areas. Overstocking a store can be a drain on cash flow. It also leaves outdated merchandise in the store, making it look a bit run down. Unfortunately too many small retailers love their merchandise too much and refuse to cut the prices. Reality is that for poor sellers the faster you take a markdown the less expensive it will be.
When looking at a retailing operation it is also important to be able to determine the profitability of your various suppliers. That is one of the requirements you should look for in merchandising computer software.
FInally, I would recommend you become very familiar with retail inventory accounting. A good retail system package can do the clerical work, but you need to understand how it works, and how decisions impact it - and your profitability.