Does Helvetica font have a character for an empty checkbox? Ie. simply a rectangle or square? If it does, do you know the character number?
Plain, standard Helvetica does not contain a box glyph. (And if yours does: be careful, if it appears to do, that may actually be the
.notdef glyph! Such a character is not suitable to use in a document!)
You can fake a box with various methods by using a graphic image, but if your document is going to be distributed as a PDF, I would strongly recommend to use the proper character code
☐ BALLOT BOX (U+2610) and apply a font that contains it.
The advantage of using the proper character is that if you copy text out of the PDF, it will be copied along with the text. This will not be the case when you use a graphic. You can also paste the character in Acrobat's Find Text field, and then it will be found as expected.
If you happen to use (a moderately recent version of) Adobe InDesign, you can copy the character out of this post and insert it straight away in your document. To apply the correct font, add a GREP Style to your paragraph style that automatically applies a character style to just this character. All the character style definition needs is a font that contains the character (Arial Unicode MS, Minion Pro, Source Sans, Segoe UI Emoji).
Only use a Dingbat or Wingdings font if its Unicode codepoints are correct; if not, then copying the text 'out' of a PDF will most likely show an accented character.
There's not an empty checkbox, but if you work on Indesign you have the possibility to create a Character Style to convert a regular space into an empty square:
From the window Stroke > Stroke Styles > New make to different strokes:
Type a space, select it and apply a New Character Style on it. Open the New Character Style options, on the sample it is called EMPTYCHECKBOX.
On Advanced Character Formats > Horizontal Scale > 255%
Apply Underline Options:
Apply Strikethrough Options:
This is the general idea, the measurements depend on the character size, 12pt on this example.
This is all Helvetica Regular from Adobe has (=no checkbox):
In theory and in practice, too you can add the wanted shape to the font. It takes 1...2 minutes in a font editor. But you have no right to call it Helvetica any more. It also could cause fatal errors because the original Helvetica isn't the same. In addition the result can be used only for personal purposes and only in your house, because you have no right to give, publish, upload to a website nor embed into a document (or a PDF) your new font which is derived from Adobe's Helvetica.
So, I do not recommend this method for other than personal learning purposes. Rather check the other possiblities which are presented in other answers and comments.
ADD: Inserting your own shapes is possible to a totally free font whose license text clearly allows you to make your own derivative as long as you rename it and include the required credits as the license demands.
I think it pays off to check if there's a good enough free alternative that allows what you want. If you are lucky you will find something that already has all wanted symbols.