eBay will suggest categories for you when you sell your item, if you type in a few words to describe the item on the category selection page and click ‘search’. You can make the best of this feature by typing in exactly what your item is, with brand name and model number (if any), so that eBay can find the best category for you.
If that doesn’t work for you, then search yourself for items like yours, and pay attention to which category most of them seem to be in (you can see this near the top of each item’s description page). Try different words and see which ones come back with the most results. You can also browse through all the available categories from eBay’s front page.
Remember that the more specific the category is, the better – use as many subcategories as are appropriate. Don’t just list your HP laptop in the ‘Computers’ category, for example – list it in ‘Computers > Laptops > HP’. Don’t worry: your item will still appear in the ‘Computers’ category, as well as ‘Computers > Laptops’, because items listed in subcategories are always listed in every category above.
Take some time to look through all the categories and get familiar with the way eBay as a whole is laid out. After all, that’s better than getting a few months down the line and finding that you still think of eBay’s category system like it’s some kind of scary jungle.