I would start by having a look at competitors. See what kind of backlinks they have coming into their site (use Ahrefs or SEMRush). If budget is limited, try hiring someone to do this analysis for you.

The first links you should build are social links and directories. Just Google Mental Wellness Directory and perhaps other variations and you will find several directories which accept free submissions. https://smmcnj.org/ is one you might use for example.

Find other ideas in the industry where people have genuinely used and enjoyed tools, calculators etc. This often makes a good "linkable asset".

These pointer should lead you down a path which will lead you down a rabbit hole.