You asked

What's the best way to initiate a conversation about sex with my child?

I don’t think that any parent relishes the thoughts of sitting their child down to discuss sex with their teenager and likewise no teenager really relishes the idea of having to sit there listening to their parents talk about sex.
You should try and not be offended, if your child looks horrified and tells you that you are embarrassing them. Remember, no matter how much they protest they will be glad that you have made the effort to talk to them about sex and it will help them to know that they can broach the subject with you again if they have any other questions.
It can be helpful to try and make sex a part of regular conversation. Mention it casually if the subject comes up on TV or while listening to the radio. It’s important that your child isn’t afraid to come to you about something they may be confused about.

More questions

It's common for teens to have questions about their sexuality...
The topic of birth control and protection can be particulary difficult to broach as you don't want your child to feel that you approve of sex at her age.
Firstly, parents should realise that they won’t be able to plan these conversations as conversations with teenagers rarely take place when and how their parents want them to.
As your teenager grows, it's important to be there to listen to your child but allow them to make their own decisions about friendships.
The essentials tend to remain the same each year but as your child gets older they may need extra items.
Your child will have an Individual education plan that outlines exactly what services your child will receive.
Any child who received special education resources or support in primary school will almost certainly be eligible for the same support in secondary schools.
If your child has been receiving extra help in primary school it is important to look for a secondary school that will suit his needs
There are lots of things parents can do to support their child in secondary school.
Transition Year is a one year school based programme between the junior and senior cycle