First, make sure your idea is properly documented. You do not have any copyright until it is recorded in permanent form.
Trademarks are expensive and the process is slow, so registering a name and illustrations may not be worthwhile until you are sure the design is going to be a hit.
If you show the concept to a publisher, whether commissioned or not, state that the ideas are valuable and given in confidence. If a valuable idea (even if incapable of copyright protection) is given in a situation of confidence, the recipient cannot use it without your consent. The idea must be sufficiently well developed for you to be able to prove that it was a valuable concept. You should also make a dated copy of exactly what you told the publisher so that you can later prove that it is your concept that is now that publisher’s bestseller.
If your publisher likes the idea, consider carefully any contract that is offered. Take advantage of the SoA’s contract checking service. If you do not think that your publisher can properly exploit all the character rights, such as character merchandising which is a very specialist field, then try to retain merchandising rights. You might also want to retain the rights to spin-off books such as colouring and pop-up books. Be careful if the publisher is going to help you develop the character. There is a risk that they could claim rights in the developed images, for example if the idea is animated by computer. Make sure that you own not only the original rights but also any material which is created from your original design. Such developments multiply as sub-licences are given. For example, the Disney version of Winnie the Pooh is quite different from E. H. Shepard’s original illustrations, and in the case of The Snowman, although the original story was written by Raymond Briggs, it was animated by another illustrator whose images were used for the merchandising. Without an agreement it would be the animator rather than Raymond Briggs who would profit from sales.
If your book and character catch the public imagination and you are asked to enter into merchandising arrangements, you can either grant a licence to a merchandising agent who will grant separate sub-licences to manufacturing companies or you can licence direct to those companies. It is important to be careful about the specific terms of merchandising arrangements. There is no room to discuss them in detail here but they have quite different considerations from book contracts, most particularly in the area of quality control.