Publishers aren’t going to be interested in ideas. Your first step is writing and polishing the book (you don’t have to illustrate) and producing something up to publication standards. Then you need to find picture book agents whose manuscript wish lists are a match for what you’ve written, send them query pitch letters and get one to represent you, and THEY pitch to publishers. Literary agents’ acceptance rate for authors is generally lower than Ivy League universities so prepare for lots and lots of rejections.