You create a budget after a deal is won.
A deal is a term we use for a work that can turn into a project but it's still in the sales phase. While negotiating with a client, you have to say what services you offer and for what price and you put it down in a deal.
Checkout Winning the deal article for more information.
You can also add a new budget directly to a project while you're on a Project page. Go to Financials tab and click the + button.
Give a name to your new budget and hit the Create budget button.
If you're creating a budget manually (not from a deal that was won), first thing you'll need to do is to add Services.
Services define what type of work you do for a client and people working on a project track time against them. You can add hourly priced or fixed price services.
For more information please read about creating services in the Sales section - it's the same for the budget.
Another type of financials you can add to a budget are Expenses. Use this option if you're focused on selling or reselling services or products (usually for a fee) rather than on time tracking.
Read more about billable expenses here.
Keeping an eye on the time spent on a budget is very useful information. It can happen you burn 80% of the hours estimated for the budget and not know about it. And maybe you're going to need more hours to finish the work than you thought in the first place.
Turn on Time warning for a budget.
Put down a threshold after which you want to be notified about. You'll get an e-mail when billable hours reach the percentage of estimated hours you set up.
You can also change the budget's currency or the date for which time period the revenue and costs are reported in.
Make sure you keep all the financial information in the budget accurate (services, discounts, expenses) otherwise your profit and revenue figures won't show the real balance.