Why agile? I think adoption start with this question but honestly many fails to answer this simple question. Answer is usually very generic like reducing time to market, faster releases and improved productivity etc. Well all these is fine but what exactly you are trying to achieve through agile adoption? Is there any dysfunction in the organization? not able to compete in market? or employee attrition rate is too high? Clear answer will help in setting clear goal for transformation.
Which framework? Can we adopt Scrum, Kanban, LeSS, SAFe or XP? Any framework is fine as long as framework is empirical in nature. Even you can ignore all these frameworks to start with and let people decide which is more suitable as per complexity in adoption. You can design your own framework but better to have someone to facilitate teaching, mentoring, setting it up and answering all your doubts. Usually people call them "Agile Coach" or "Transformation Agent". Selection of framework depends on work and organization complexity but to reduce complexity, better not to choose complex framework.
How to select Agile Coach? Better to hire someone good in facilitation and understand software engineering. Someone with lots of certificate? may be if you don't know anyone because there is no other way to filter but better to speak to their customers to understand more about individual behavior and approach. Hire Agile coach and not "Enterprise Agile Coach", "Executive Agile Coach" or "Team Agile Coach" etc. Agility is also about getting away from complex organization design if coaches themselves start promoting hierarchy then what they will bring for you is a big question. Coaches should facilitate rather preach to ask question around facilitation techniques. Someone having good knowledge about agile, facilitation techniques and software engineering can be a better choice. Must be a good listener.
From where to start? Start from wherever you are. Yes you need a leader within organization who understand why agility. Better situation if leader is very senior in hierarchy else someone very influential. Even that's not the case then adoption will slow down but still possible. Start small rather than big bang implementation but once you see success and win confidence of leaders then better to go big bang. Ensure transformation not restricted to software development but happen in all supporting department like human resource, payroll, marketing and general administration as well. If not then agile organization will not be possible and such cases people may go back to old path because required support to sustain agility will be missing. Ensure there are leaders (not managers) from every field to lead and support transformation to see better result.