Across the IF app you'll see the buttons below, or a subset of them. There are lots of ways to get started, but it's probably best to make sure that you complete your Account Information and your Study Interest Profile first - both of these are accessed through the Profile button. It's a good idea to keep these up to date as your ideas change - you can come back at any time and do this.
Once you've done this, you can create your Career Interest Profile by inputting your text, via the Text button.