You can talk about when something began with the phrase "___ first start to ___":

I first started to suspect that there was something wrong when I was cooking dinner and realized that I hadn't heard from her all day.

When did you first start to think that you might want to be a doctor?

People usually use "first start to" with words that describe thoughts and emotions like:

• realize
• think
• feel
• notice
• wonder

Adding "first" is optional. You can also leave it out like this:

I started to suspect that there was something wrong when I was cooking dinner and realized that I hadn't heard from her all day.

But saying that something "first started" to happen emphasizes that this was the beginning of something that continued for a little while.