Here are some references to specific technically information on how to start with creating and building a binding: Create jar for only one binding in the addons repository - #4 by hilbrand
Yes create your fork (you already did this) and start from the main branch by creating a branch for you binding (the new binding documentation I linked above to, describes this).
When you’re completed your binding then you can create a pull request and it will be reviewed for inclusion. You could also add your binding under the marketplace topic to allow others to install your binding.
Some specific comments. Currently openHAB 4.0 is in development. Because there are some breaking changes 4.0 build bindings won’t run on 3.4. But there are ways to build a 4.0 binding to be compatible with 3.4. See Migrating addons to v4.0 - list of necessary changes available? - #6 by hilbrand
You could start from the 3.4 branch if you want to develop specific for 3.4. However if you want your binding to be included in openHAB you need to develop for 4.0. Therefor it’s recommend to start your development from the main (i.e. 4.0) branch.
This is all very brief information. The best way to start is read the documentation, look at other bindings (for example look at merged pull request. There is a lot of knowledge on how things work) or search/ask on the forum here.