in michigan it’s super easy to access hrt thru planned parenthood, they’re a great place to start. it’s never too late to start and pass, and 21 is still relatively young (even though ik it may not seem like it). if you have a queer community center near you they often have resources to connect you with other trans people and get information on lgbtq friendly providers and legal processes.
Congrats! And don’t worry, It’s never too late! If you’re looking to medically transition 21 is a perfectly fine age to do so. You’ll need to contact a doctor to discuss your options. Also speaking to a therapist / trained counselor would be a good idea for dealing with any concerns and difficulties with being trans.