Budgeting means several different things at different times during your project. Some budgeting you'll do; some is done by others.
You will certainly need to think about the total funds you feel you will be able to commit to the project. This will give important guidance to your architect as he develops designs that aim to accomplish your program. It may also inform recommendations to make changes in the scope of that program. Likewise, you'll need to think ahead about the timing of your project's cash-flow needs and how you will meet them, as well as any possible financing you will need to obtain.
There are several ways for you to develop a budget for your project. Your architect may make cost projections to accompany designs, so that you may use that information when making a decision to approve any particular design. Alternatively, some architects will recommend that you or they retain an independent firm known as a "cost estimator." These firms understand current construction costs in your region and make calculations of cost based on your architect's design. When you are hiring your architect, ascertain which method she will employ, and its cost.
Following this, when your design is put out to bid to various contractors, they will provide a construction budget that will become the basis of your contract with them.