I don't really know for certain but I think I might be. Like, I've never liked hanging out with girls and stuff but I'm really worried because what if I only think I feel this way because several of my friends are trans. How do I actually know for certain?

Additionally, I was talking to my mum about how I want a penis (because I know that much) and she's like

"ARE YOU SAYING YOURE TRANS"

"Cause I don't think you're trans"

Even though she supports all my trans friends.

How would I know for certain? I don't want to move forward then move backwards I feel like I'd get too much shit for that.

No one can tell if your trans but yourself. Make a pros and cons list of why you would like to be female or male. You might find that you like both and would like to sit somewhere in the middle. Labels are really important, what makes you happy is

Hi. As someone who is trans I still struggle with this a bit.
But no one can tell you your gender other than you. If you feel like you may be trans have you thought about seeing a gender/LGBT counsellor and discussing your thoughts and feelings. There's also Qlife the national LGBT helpline on 1800 184 527. Theyre available by phone or webchat between 3pm and midnight.

Hi, trans person here! :)

A big tip that helped me know for certain I was trans: cisgender people do not think that much about their gender identity. This personally really helped me realise I was trans.
I’d recommend seeing someone who specialises in LGBTQ+ people and experimenting with what makes you the most happy. Gender euphoria is a very strong indicator that helped me realise.
Unfortunately, it does take time to figure it all out. I spent a lot of time wishing I could just know the answer — but the time will come, see what makes you feel good about your gender!

Also you mentioned some of your friends are trans — it's natural for LGBTQ+ people to gravitate towards one another, even without realising! :)

Best of wishes! 🏳️‍⚧️