Whether or not, and who you disclose your ADHD to is your decision. It is often best to disclose to you immediate superior as presumably this is the person you report to most often and for whom you have to meet deadlines.
As for 'how to', honesty is the best policy; arrange a meeting and explain that you have been diagnosed with ADHD. It is often helpful to employers if you can explain what this means in practical terms; for example "it is difficult for me to retain large amounts of information provided orally." If you can then offer a solution to the difficulty, it shows your employer that you are prepared to take responsibility for your ADHD and make things work as you value your job and position in the company. An example solution to the above mentioned problem could be "please email me your requests so I can keep better track of what needs to be done.
If you need any stationery to make your work station more manageable (colour coded sticky notes, monthly planner etc) decide if you are able to purchase these or if you would like to approach your company.
Best of luck,
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