I'll second the Pond Guy forum.  Lots of good people and information there.  Here are some things to get you started.  First, find a way to keep the water moving.  Aeration, pond circulator, windmill, etc.  Those will help keep the water from stratifying and being overly warm on the top.  Second, get some dye to color the water blue and/or black.  I get it online but most farm/ranch stores will have it.  My 3/4 acre pond takes about a gallon of treatment every month from April through October.  To kill the existing algae look into copper sulfate.  I've used it with limited success....mostly because I don't take the effort to apply it correctly.

Cattails are easily controlled with glyphosate.  You can get special aquatic glyphosate which doesn't have any surfactants in it but since you don't care about fish just use RUP.  I have fish and still use RUP.  I doubt you will eliminate the cattails but you can limit their spread.

The biggest issue with my pond is keeping corn leaves and husks out of it in the spring and fall.  They introduce too many nutrients and create an issue.  I need to address the problem ASAP as I have some unwanted algae growing.

If you want to get real technical look into applying Aluminum Sulfate (Alum) to the surface of the pond on a calm day.  It will take the colloidal clay particles in the water and also bind to the phosphorus and sink them to the bottom.  Your water will be crystal clear for a few days until a new algae bloom begins to grow but it will be much more manageable.  I did it 6-7 years ago with great results but it takes some research and effort to apply.