All of the mod options are going to require something that is called a manifest. For those who don't know it is the .json file that stores the basic information about you Mod and gives it to SMAPI. While each mod type has a manifest they all look a little different so lets take a look at a Content Patcher Manifest first.
Here is out example manifest.
You will always start all of your .json files with a { and your last line will always be the corresponding closing }.
From here the explanation is pretty simple you have
Your Mod Name
Your username
Version Number - this is important to change everytime you update! If you don't SMAPI will NOT register that your mod has been updated.
Description - Just a description of your mod dose
UniqueID - this is important to make sure that mods don't collide with eachother. I find it safest to add my userid and my mod name for the Unique ID
Update Keys - This is another one that is super duper important if you do not put an update key the user will not be notified via SMAPI that their mod needs updated. In some cases this can be game breaking!! To find your Nexus update code it is actually the numbers in the top of your mods link this generates after the first step when you are posting your mod
Content Pack For - this is what tells SMAPI what framework your mod is using and will be the biggest difference for the manifest in the various frameworks.