John from Hillsdale, Ontario wants to create a storage area in the attic of his garage and wants to know just what he needs to shore it all up. His photos showed that what is there now is not designed to hold any weight at all.
The short answer is that he needs to have a contractor or structural engineer take a look at his garage in person to properly size a support.
The long answer is that he has to look at far more than the beam. A beam can support the ceiling/floor joists (just like in the basement), but there must be adequate posts to support the beam. And then there has to be an adequate footing to support the posts. In fact, you have to trace all the weight from the top of the garage (snow, wind, roof, floor, ceiling, stuff in storage, people) all the way down to the soil. The posts in a basement appear to be just sitting on the concrete floor, but in fact they are sitting on specifically sized footings that are adequate to distribute to the soil the weight of everything above without sinking. That is why an engineer's plan is worthwhile, even if you want to do the work yourself. Otherwise you may end up punching a hole through your garage floor.