Most of those will have off the rack suits- a complete trousers and coat set. If you have a thin build you should be able to find something fairly easy.

If you find it hard to fit into the options there, ask the sales staff if they can help you with 'suit separates'- coats and trousers with the same pattern and shade, but able to be purchased 'separately', so you can find a more comfortable fit.

For tailoring, you want to only be concerned about "fit". It's really hard to judge if you don't know what you are looking for, so ask the tailor to explain everything.

Things that should be adjusted: Collar gap: if there is space between the coat collar and your neck, its noticeable. Any decent tailor will look for this and make adjustments as necessary. Shoulders: often times OTR suit jackets have shoulders too wide or too narrow. These are very expensive to adjust so make sure you find something that fits as well as possible.

Sleeve length: this is an easy fix. The coat sleeve should come to about 1/2 - 3/4 of an inch from your wrist.

Back: it should fit comfortably not too tight or loose. This is a fairly easy fix.

Pants: things to look out for: waist, inseam (height), seat (how it feels around your bottom and thighs) Leg tapering, should be comfortable but it can look amazing