If you have items you sell at retail or wholesale, you probably will not have bare shelves at the end of the year, so you have to count your inventory. I recommend that you make a list of all your items and their unit cost. Then just count the number you have in stock and multiply.
Please note that you use the cost, not the selling price. That is, you want to know how much cost you have in each item, not the amount you expect to get when you sell each one. Ignore financing, even if you have not yet paid your vendor for an item. You will pay eventually, and for these purposes, count everything you actually have in your possession.
Try to get this done either after the last day of business in the current year, or before you open for business on the first day of next year. If it takes a day or so before you get around to it in the new year, you should adjust for items you had on hand but have sold before you got to your count. The longer after the opening of the new year, the more adjustments you will have to make, so try to be as timely as you can.
Besides resale items, you should also count your supplies. Everybody has some supplies on hand at the end of the year, even people who are in a service business without regular resale products. Do the calculation the same way: item description, number on hand, cost, and multiply the number and the cost per item.
Most people dislike taking inventory (a dusty warehouse, during the cold of winter, takes a long time, is boring, etc.). Admittedly, those are valid feelings. But you can enlist the aid of all your employees and family members. Many hands will make light work. Go ahead and get this job scheduled.