You can make your own digital coin by:
- coding it from the scratch
- changing the code of the already existing coin
- using CoinCreator
The are hundreds of digital coins at the moment with Bitcoin the most popular (84% cryptocurrency market share).
If you want to change the Bitcoin itself you have to join the team of developers, make some proposals and eventually your ideas will be included in the new bitcoin client release, because bitcoin is constantly changing, improving. There isn't much original code left.