Heading into a major renovation without a detailed plan, materials schedule and a clear budget can turn what should be a gratifying experience into a nightmare. Preplanning is the key to success. Take your time, if you don't have the ability to create detailed drawings of the new layout, then it may be wise to have an architect create some plans for you. There are lots of items to be considered, plumbing, electrical, structural changes etc.
You should spend time at home centers or specialty showrooms so you can determine what kind of fixtures, cabinets, counter tops, flooring etc, that meet your needs, style and budget. Remember, quality of these items varies tremendously, so do the costs, so get a handle on those items early in the process.
Create a "schedule" or list of the exact items you want in your renovation. The more detail, model numbers, colors, accessories, the better. Even have samples of the flooring, trim, finishes etc. Many places will loan you a sample cabinet door and small samples of counter tops.
The advantage of having an architect is that he/she can take your selections and work them along with the unseen requirements into a complete "Scope of Work" that can be used to solicit quotes from contractors. The architect may also help you in constructing a request for bid that will include items such as time schedules, schedule of payments, and final hold-back (10% is common) until a final acceptance inspection is completed. The business aspect of a project like yours is very important since you are probably talking $30,000 to $40,000 or more.
When selecting the contractor, vet them thoroughly, check references, go look at some of their recent work and talk to their customers. Don't be shy about it, I have seen and cleaned up after too many botched and unfinished jobs because the customer fell for a smooth talker and never checked them out professionally. Be very cautious.
I could probably write a book based on your question, but I hope I have covered a few of the bases. Feel free to ask follow up questions or more specific questions. Good luck.