Youtube is a great resource, but there are also plenty of poor or inappropriate examples of Tai Chi on there (you don't have to be an expert to post a clip online), so check the feedback before trusting any videos.
The "Lady in Pink" is well known for the 'Simplified' 24-form (the most popular across the world)
(...she has trained for many years, so don't let her great flexibility put you off)
This Shibashi 18 posture Taiji Qigong video is by Shan-Tung Hsu's Hong Kong "sister".
(Chinese with English descriptions. 1st 20 mins = breakdown, 2nd 20mins = flowing movements)
Training with an Instructor is the best way to ensure you are performing the moves safely for you, especially if you have an existing health condition.
Here is a (50 min) talk by Dr Paul Lam on his "Tai Chi for Health Program"; Founder of the 'Tai Chi for Arthritis' form and clearly a fit 60+ year old in 2009 despite being an Arthritis sufferer from the age of 13. www.youtube.com/watch?v=co1uxOFJ8Mc
World Tai Chi & Qigong Day photos from Sat 28th April 2018