I'm start to learn about blockchain and crypto and started a private project.
so i have a doubt, how can i soft fork it on block X (to ajust reward for example)
You do have to update the nodes software to behave the way you want to after block X has been reached.
Note that you can't adjust the reward in a soft fork. This would require a hard fork.
A soft fork is defined by being backwards compatible.
New blocks from updated nodes still have to be valid for non-updated nodes.