This guide assumes you are setting up AWS for the first time either for personal use of for a small business (couple of users). The steps will also scale to a large organisation but it's likely that you will want to think through a more complex structure for grouping users into accounts and the permissions they have.
From the top-right drop-down, select Billing and Cost Management, then Budgets, then Create budget. Here you can define a budget or use a premade template. When starting out you may want to try sticking to the Zero spend budget for example.