If you already have plans for your kitchen you can approach showrooms or independent kitchen designers and ask them to design and or to apprise the kitchen you want according to your plans. if you don't have plans you can either take measurement yourself or ask a kitchen designer to come to measure. once you have a proposal from a designer and you like the proposed design and pricing, you should retain your designer. if you are not satisfied you can ether ask the designer to provide you with additional options and\ or see proposals from other kitchen designers.
You should work with a kitchen designer which you feel is listening to your needs and is capable of designing a good looking and well-functioning kitchen. communication is KEY for the success of your project.
If the designer is also the kitchen supplier and you feel the price is attractive, the quality of the product is good and you like the look and feel of the design then you should retain the designer or if the price is too high you can ask the designer if there are ways to revise the design in order to accommodate your budget. Or, you can see proposals from other kitchen designers who can also supply the kitchen.