Search for terms related to your website on Reddit’s search. Check out the subs where the results are posted, that should at least give you a base from which to work. A lot of subs won’t take external links and even more filter all of them and approve them manually.
If you want to promote on other subreddits make sure you are using an account that is aged a bit and has some karma points to it. Some subreddits won't allow you to post otherwise or if they do the members will not trust your information that much.