Old John,
Trees on dam...don't remove trees larger than 4 inches diameter. Trim them. Smaller trees, remove. The dam is a water management structure, and has been engineered. Decaying root balls from big trees can compromise the structure.
Leaves decompose, adding to organic buildup on your pond's bottom. Aeration can help. Read up on that here, lots of threads, and you can call some our quality advertisers for advice specific to your situation. Go the to the resource guide for info.
Cattails are a problem, if you think they are. Otherwise, cattails don't pose big biological issues unless they cover most shallow water, and much pond periphery. So, it's really a personal choice, whether you like them or not.
The last piece of advice...go through this forum. There are thousands of posts, thousands of topics. You will be surprised and amazed at what you learn.
Plus, a subscription to Pond Boss magazine is a good idea, too. It keeps you up to date on all the latest, with timely information.

Teach a man to grow fish...
He can teach to catch fish...