Monkeyface's advice on having a business plan is very sound.
You need a plan and, when making that plan, you should think about a few things in particular.
First, are you going to have a brick-and-mortar shopfront, an actual, physical store? If you do, it's going to suck up a *lot* of money in the fit-out and in the rent. You're going to have to sell a lot of products to just cover those baseline costs before you start to make any money to re-invest, let alone pay yourself a salary.
Secondly, are you going to have a website? I assume so, as most people do. A good website, particularly a decent one set up for e-commerce, can cost a reasonable amount of money so it pays to try to defray that by doing some of the stuff yourself, such as photography. HenryC could talk about that with much, much more authority than I can.
Thirdly, marketing - how are you going to do it? It's all very well having some products and having a business, but unless people know about it, you're not going to sell anything. Some people are great at getting free marketing as they've developed a network of great contacts, others give away plenty of stuff and hope for referrals and publicity that way, yet others pay for advertising, and of course there's internet fora as well. Most people, of course, will use a mix of approaches and you have to sort out what you think will work best for you. You definitely need to do something, though, because otherwise you'll be sitting around twiddling your thumbs.
Next, what else are you going to stock? Just a few jackets and suits? What about accessories? Shirts, ties, socks and so on. Frankly, while you potentially stand to make more overall profit per item on a suit or jacket, I think that you'll make more profit as a percentage on shirts and accessories. When people come in for a jacket, won't they be looking for a shirt (or shirts) and a tie (or ties) to wear with it? You could probably make another 50% profit on getting people to buy some accessories along with their bigger purchases.
Finally, are you going to sell off-the-rack clothing and accessories, or are you going to offer only made-to-order clothing? If only MTO, then cloth is a non-issue because people placing orders will, of course, choose from cloth books used by the manufacturer (or cloth books for mills that you can order from and send the cloth to the manufacturer, if they're happy to do that). Of course, even if you're selling MTO clothing, it's very useful to have a range of clothing around so that people can try on items and see how they like particular cuts and basic sizes. MTO clothing is much cheaper to go with as a start-up option because you don't need a large inventory.
I hope that I don't sound patronising - the above are just some things to think about (and you may well have thought about them already) while working on your business.
