I hate T-Com with a vengeance.
Just over a year ago I tried to sign up for DSL with 1&1.
T-Com actually refused to allow them to supply my service and sent me their own bill for DSL.
It took 3 months and far too much hassle before they cancelled my non-existent (
T-Com) order and refunded the set-up costs which they just took out of my bank account along with my normal telephone bill costs.
In December last year I tried again and this time it went through without too much problem.
I do like 1&1, I use them for most of my internet services and they offer good value and a pretty reliable service.
I have the 6MB DSL but they now offer 16MB.
As to how to do it:
Have a good look around at what offers are current.
The PC magazines are usually full of this stuff.
After selecting, you just sign up - usually a minimum of 12 months contract.
You will receive a DSL Router and a few cables.
Do you already have ISDN or normal 'analogue'?
If ISDN, you can choose to keep this alongside ISDN, but the cabling is a little more complex.
This sounds easy and over simplified, but it really is that easy.