It's very difficult to say without knowing the quality of H and whether it's mixed homogeneously or not. Fentanyl adulteration being very prevalent these days also warrants the use of a test kit such as
TKP's fentanyl-test-kit.
If in fact you do have Heroin and not Fentanyl I would either insufflate or smoke depending on which type you have, #3 (mostly Europe) is better for smoking; $4 or Tar (mostly US) better snorting. IV/IM for first use is more than inherently dangerous, not even getting into other issues surrounding that ROA.
a couple golden rules:
*Always have someone with you that's not using, better yet someone familiar with the batch of H you're going to use.
*Always have that person armed with and well versed on the use of Naloxone. Availability is getting better everyday and there's no excuse to not have it these days.
Strongly consider using a pharmaceutical opioid in a standardized dose instead if it's just the "experience" you seek. It's not often I'd say that unless H is also being considered.
Obviously there's far more to it but hopefully you can continue to study up before jumping into this.
For dosages I'd refer to the site below, but it's near impossible to eyeball 5-15mg. Some will say things like 1/2 a match head but without knowing the purity you're going to be in the dark. This is where it's always best to build up progressively using minute quantities till you know the quality of the H you have.
https://wiki.tripsit.me/wiki/Heroin
Also since you mentioned using it for the first time:
Should-I-Try-Heroin-
~first-time-using-heroin
The above info is just scratching the surface but one must start somewhere.
I will now defer to my right honorable core BDD'ers as this hasn't received any replies in OD as of yet.
Welcome to BL Swan and I hope you use an abundance of caution in this venture