it’s quite simple:
on the render method you should keep an eye on the bind(this)
<select onChange={this.yourChangeHandler.bind(this)}>
<option value="-1" disabled>--</option>
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
</select>
and your handler is like:
yourChangeHandler(event){
alert(event.target.value)
}