Best way to attach a steel t-beam to a wall?
T-beams can be connected to walls by welding a flat plate onto the end of the T-beam, drilling holes in this endplate, and then bolting through those holes onto the wall.

Alternatively, brackets can be bolted to both the wall and the beam. Example of a simple connection. Example of a full moment connection. (Both examples are for I-beams, but the principle is the same.)

In your situation, it appears you are talking about significant structural works (i.e. you are talking about walking on the platform). The idea of a DIYer attempting this worries me significantly. If you don't know methods for connecting a T-beam to a wall, I suggest you are unlikely to know how to correctly size the T-beam, or design a sufficiently strong connection. I advise you to consult a structural engineer.