Welcome to the fold! This is my first Porsche but I have been riding bikes since college where I learned on a Ninja 750, so I am used to getting tossed into the deep end. I started out with a lowly 5-speed manual Mazda Protege back in day with new wife and limited funds (my first "real" car after my $200 rusted out junk car) and it was a very easy manual to learn on. Since then, I have had various cars from CJ7s to SCCA Vettes (383 Strokers) & ZO6s. --- all manual. It is true, a manual is a manual, but I do find the Boxster to be rather unforgiving, has pretty good power (if you punch it) and easy to stall at times.
I agree with the premise of learning on something less forgiving first to get the knack of a manual in various situations, uphill, downhill, parking, stop signs, rolling tendencies, and the clutch to gas pedal release harmony. It takes some time to learn and you might be slightly more aggressive in the Box just because you want to "show it off" and you could grind some gears and stall it alot if you are not paying attention. (not good for you or the car)
Just a few Suggestions:
1. Turn the #$%#$% radio down or off. Shut up your gf or leave at home.
(learn to "listen" and feel the engine and need to shift, you should get so good that you never have to look down or even peek at the tach, you can hear and feel when you need to shift)
2. Find an empty shopping mall or grocery, something and practicing starting and take-off at "lights" or "stop signs" - practice steady control. Don't RIDE the clutch or keep it engaged. Learn to shift to neutral and use just your brake at lights, and only position it into 1st just before the light change. No need to MASH the clutch at all times!
3. Just like a bike, don't start trying to gun it or race anyone. (at least not when trying to learn!) Learn with good control, discipline and by being slightly defensive when driving a new manual.
4. The biggest thing is control and feeling of the shift. It will come but I don't know if I would have tried to "learn" on a Boxster. My clutch at least (it is relatively brand new) is pretty hard and tight. Nowhere near anything like a Civic or a SCion I wouldn't imagine.
5. If you do "Get into trouble" (like a bike again) --- engage the clutch and regain control before you get in over your head. It certainly is not rocket science, it will just take a little time...(good thing? Once you learn to drive a Box manual, you can certainly drive nearly anything else.....IMHO)
Practice, Practice, Practice.
All the best and welcome.
Cheers!