The easiest way to do it is to drag a bullet in your characters bullet animation. Let me show you how.

Let us start by creating a simple Platformer game. 

After clicking on create it will take you to the game world where you can edit the scenes. Click your character to find the character settings.

On the options panel, drag your selected bullet animation.

When you preview your game, you will automatically have the option to shoot by pressing the spacebar. 

Now you will need to add a shooting button so your players can shoot even if they are using their phone, ipad or any similar device.
To do that you need to go back to the game mind map by pressing the Menu Editor button and double click the World UI. Now you will have to drag the character button to your desired location.

And select a button you want to use. To do that you need to click your character button and drag the your image to the image section.

Then select a name and select shoot as function.

Now when you preview, you will have a button to click to shoot bullets.

Now that we are done with the easy way, let me show you how to do the preferred way.

On a Platformer game, let us drag our bullet image or animation to the Object portion of the Buildbox wheel.

And then press the Activate Connections Mode button found on the upper right hand side of the scene editor window.

While connections mode is on, connect your character to the bullet and drag it to the character. 

While the bullet image is selected, click add component on the lower right hand side and select spawner.

Don't forget to change the Preset to Character Bullet. And at the bottom select single shooting.

Now when you preview your game, you should be able to see something like the below.

Or you can change the Spawn Action to Shooting and the Spawn Rate to 2 to get multiple bullets. 

And if you wish to, you can change the X/Y/Angular variables on the bullet to get the trajectory/speed of the bullet you would like.

Now let us make a button for our bullet. In the World UI, drag a character button to your desired location.

Then while the character button is selected, change the name, drag the selected image to the image section, then select function as Shoot.

Now when you preview your game, you can already use the button you created.