My child says they are transgender, how do I cope with this?

With the love and support that a parent should show to their child regardless. No matter your child's gender identity, your role as a parent does not change and what your child looks to you for does not change.

Let them talk about it and express their feelings openly. If you have any questions, ask them, but always in an empathetic and tactful way. Make sure they have all the support they need. Acknowledge their identity and their needs, support their journey towards being the person they really are. If you're new to all of this, do some research about what it means to be trans, maybe look for support from an LGBT associations - many of them are used to supporting the parents of LGBT people and helping them understanding and coming to terms with it.

Ask your child what they want you to do (like call them by their prefered name and prefered pronouns

At the risk of sounding perhaps a bit harsh, perhaps the better question would be "How do I help my child cope?" Perhaps by helping your child, you will help yourself. Helping others is in of itself quite empowering.

Accept them as they are. Try to understand they can't help it as sexuality is something internal which is can not be changed.

First, it's important to realize that it's not your fault nor their fault that they are this way; neither of you did anything wrong that "caused" your child to be Transgender. Like thousands of others, that's just the way your child was born (ie just like your child was born with a certain personality that neither of you had a say about). That being said, having a Transgender child can be both confusing and difficult for both you and them. Your role in respect to your child is to realize that he/she is probably going through a super difficult time and needs and craves the stability of your love and acceptance for who they are - now more than ever. Of course for yourself it might be helpful for you to join an online or in person "parents of LGBTsupport group. " this can help you process everything, clear confusion, and give you hope for what can initially look like a negative future. Of course, a healthier,less confused parent will mean a healthier child as well. Like all children, your child too can succeed and be happy with the proper love and acceptance. Good luck!

Talk to your child to find out why he/she made the decision in the 1st place and let him/her explain to you on how they feel on doing so.

You should try to talk to him about it and seek professional counsil if needed. Most importantly don't pressure him or her to change.