Write it. Edit it. Submit it to agents that work in children's fiction.
Unless you intend to do the illustrations yourself don't send illustrations along with the manuscript. Publishing companies generally prefer to match text to illustration themselves.
There is also the small press options if you want to skip the agent. Then you have the issue of low circulation and publicity.
One of the most important things to start of with though is figuring out who your audience is and making sure it's age and development appropriate. Will it be dependent on illustrations? Will it be tactile? Will it be a read along? You have to map out the entire thing from start to finish. Only then should you think about submitting it.
Plus, make sure you're not over-saturating the market. If it's already been done, don't re-do it.
And if it get's rejected by 30 agents, don't fret. Just keep submitting (to different agents.).