One of the easiest ways to improve the effectiveness of your ads is to find which products have sold the most in the past and then spend more money showing ads for those items.
This is important in making Google Shopping work for your online store.
Check your past sales records (if you have them) to see which products have sold the best.
You can also use a tool like Google Analytics to find top-selling products, learn how many people buy them after clicking your ads, and see how well your ads are making money.
Another thing to think about is how much profit you earn from each product.
If you group your products by how profitable they are, you can set your ad spending in a way that makes sense for each group.
Spend more on the items that earn you more profit.