What brand you buy or where you go is lessimportant than the actual fabrics.
Typically, you'll be wearing a wool suit. Wool gets more expensive based on thread count; you'lloften see this reflected as "Super 100s" or "Super 120s"; it ranges all the way up to "Super180s" I think. (That is the highest I have seen).
The higher the number, the finer the fabric, and of course the higher the price. However, there is a notable comfort difference as well. If this is a casual 'once-in-a-while' suit, you'll do fine with 120s.
And a 120 from Boss, Canali, Mens Wearhouse or the pricey shit like Loro Piana is all the same. The highest you'll usually see 'on the rack' is 140, to go above that you will probably have to go to a talior.
Another consideration is the cut. Depending on your body type, ask about the cut. If you have a trye V-shape with skinny legs, you;re looking for the 'European cut'. If your upper body is a little bit more square or you have "squatter's ass", ask about the American cut (since I am assuming you are not having this custom tailored).
Should give you something to start with.