I am totally confused about gedcom.
As said by others Gedcom itself is simply a means of transferring information from one program to another so that the information can be presented in a readable format similar to or exactly like the original.
I want other people searching their families to be able to see the trees I have created for me and my family, which are on myheritage. Do I have to do something to make the trees on myheritage visible to other people (including people not using myheritage), or does it happen automatically?
MyHeritage restricts access to any trees you have on there to people you give permission to. although other members of Myheritage can find your tree they have to contact you for access permission. It's not automatic.
Opening a free account on Ancestry may open your information to a wider audience if you make it public and searchable. Again it is restricted to members of that site - Although you could provide the tree invites to people you wish to allow access without them having to take out a subscription.
The sites suggested by Gadget should provide a basis for some research but spreading the information about generally requires some assistance and action from you.