How would I go about doing this...

First of all, Welcome!
The idea would seem ok, but there are just a few flaws. People would hate falling off the tree too much(?). It would be hard to optimize.
Oh and i saw that you are planning to put a lift, lifts aren't used in tf2.
As long as you can create something fun and enjoyable, go ahead

IzzyVonSchmizzy said:

Is there anyone that would be interested in playing this map?

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if you make it fun and enjoyable, then yes.

IzzyVonSchmizzy said:

And, I'm not very clear on how I would be able to go about creating the tree itself.

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displacements for the rought shape and some models for details (twigs etc.)

Honestly, this would be a very difficult map for a beginner to finish. The tree would require either custom modeling or some expert displacement skills, and the layout would require good optimization knowledge and experience.

I'd recommend trying a different layout or theme, even if you don't release it. Just something to get used to how Hammer works, how the basics go together, how it looks in game after you build it, etc. Then you can come back to this idea once you get your chops up and really make it match your vision.

KOTH as a game mode is a good choice for a beginner, though. Perhaps you can modify the idea and place it inside a wooded grove, with cliffs as the room "edges."

Make sure you take a look at this before you start: http://forums.tf2maps.net/showthread.php?t=4674

As for interest in playing, if you can make a map visually appealing AND balanced, you're well on the way to having loads of people want to play the map.

Good luck!

Kep