The NIH is one of the 'easier' federal agencies to apply to because every year it issues an Omnibus Solicitation, requesting investigator- initiated topics. This means that rather than telling you exactly which projects they will fund, the NIH asks you, the investigator, to come up with the ideas.
As long as these ideas are related to Human Health, have the requisite level of Technological Innovation and Commercial Potential, they may be appropriate for SBIR/STTR. However, you still need to do some homework to make sure your idea fits within the research interests of the NIH's Institutes and Centers. So before you put a lot of work into developing your proposal, there are a few things you can do: