Your new year resolution includes "Write clean Apex code". So you run PMD with a full ruleset and get shocked by the number of violations. You drop the resolution in a blink.

Don't boil the Ocean

Even a journey of a thousand miles starts with a single step, so let's break down the task into manageable chunks to divide and rule.
There are 2 dimensions you can use: Type of code and priority levels. Using them you can turn your Clean Code journey into manageable stages.

Code Types

• Legacy code: all code that doesn't fall in any of the two other categories
• Changed code: code that needs change due to business requirements
• New code: new code written for new or changed functionality (applies to copy & paste too)

Priority Levels

• 1 = security and performance, will fail build
• 2 = bad code, will fail build
• 3 & 4 = hard to maintain code, will generate warning
• 5 = ugly code, will generate hint

PMD rules for code types should have different priorities. A different number of tests will fail a build:

• 11 for legacy code (all around performance and security)
• 33 for changed code
• 44 for new code

This will require to run PMD with different rulesets on subsets of your code base