I do a little in all most every scale. (Building a large N scale layout, collecting HO scale, and working at an O scale museum.)
My input would be this. Base your scale on what you want to build. For example, when I went to play what sort of layout to build I wanted to build a prototypical representation of a real railroad. HO scale was considered, but was too large for the room. N scale made the perfect choice as I was able to fit every thing I wanted.
Some things you want to consider, what will you be doing with the layout. Are you circle burning? (Letting the trains run and enjoying them roll by.) Or are you operating. (Taking the train out of the yard and spotting cars at industries.)
Next, do you want to be freelance and layout track just to have fun with it. Or do you want to pick a railroad and location and try to capture the look and feel of it.
With those thoughts in mind, you can start looking into track plan's. Look at the different scales and see what you can come up with to fit in your room. (Remember, the larger the scale, the less you can fit in a room. So smaller scales are better for smaller rooms.)
At this point you could draw up a rough track plan in both O and HO for the room you want to build in. The see which you like best. From there on it just a matter of collecting the trains and laying the track.
As for manufacturers, HO has quite a few. Kato, MTH, Athrun, Atlas, Bachmann. All of which have their pros and cons. All of which, I have at least one model of. Bachmann tends to be some of the cheaper models, in cost but not quality. In other words, you get a lot for a little with Bman. While MTH is at the opposite end of that spectrum. High quality models, lots of details and cool features. But you'll be dropping a few Ben Franklin's down for one. The other three, Kato, Athrun and Atlas are all pretty good.
Id get in to the details of the O scale stuff, but all my O cost around $1000, so I dont mess with it too much.
