How to fork Bitcoin and build own cryptocurrency

To start a new chain, use a genesis generator, apply the new genesis to the source, and remove the checkpoints.

If you want to apply new/different rules, be prepared for a difficult task. Changing even the slightest protocol rule will most likely affect all other rules because of the complexity of PoW.

If the only thing you want to alter is the maximum coins, this should be relatively easy to alter in the source.