So me and my friends just started and I'm the DM and trying to help fill out character sheets. Where do cantrips go and where do spells go?
Each character sheet has a different layout - there is no standard, nor is there a required layout.
However, spells are not something available to all characters, so they are generally included on an entirely separate page.
If you use freely available character sheets ...
The third page is where the spells and cantrips go. On pages 111-113 of the downloadable Basic Rules you will find three pages of character sheet information. The first has the ability scores, race, class, traits and equipment; the second background, allies and treasure;, and the third one is all for spells. These are the same character sheets (3 pages) that you will find after page 316 (last page of the index) in the Players Handbook.
You can also download "form fillable" or "printable" blank character sheets from WoTC.
Note: the link to that third/spell page works in Chrome, but I have been advised that not all browsers function that way.
There are a large number of character sheets available. Some are official, such as the Adventurer's League sheets (for organized play), but thye run the gamut from pdfs, to pre-printed sheets, to digital to scribbles on notebook paper. Each group (and often each player) will have their own preference as to how to record and display their character.
However, Wizards of the Coast does provide a number of options for 5th edition, should you be looking into something more official or professional looking. Their collection can be downloaded from this page.
If you select the 5th edition Character Sheets, there is one specifically used for recording spells, called Spellcasting Sheet (Optional), in both print and form-fillable varieties.