To boost any individual item, you need to include the following in its inventory metadata (see Defining your Inventory for more information on how to add to inventory metadata):
"value" : "high"
If the standard strength of boost provided by including this metadata is not adequate for your use case, please get in touch with Support to discuss more advanced boosting rules and logic. We can both change the default boost strength and set up rules that allow you to specify different boost strengths on different items. Setting up the rules correctly requires some site-specific analysis, and hence requires getting in touch with Support.