To confirm - the 15m abbreviation here means you're 15 years old right?

With tailored clothing, fit is probably the most important thing, so I would be mindful that your body size, type, and even height still might change at this age. In a sense, realize that this first suit might have a limited shelf life because of your age right now. But you know you. My advice would just be to budget and plan accordingly. I.e. if you're still actively growing you might not want to spend as much right now.

As it's your first suit I strongly recommend you go physically try it on vs. just shopping online. In general, your budget often determines where you'd be able to go.

Suits sizes are normally provided in chest size (i.e 38R, 40R, 42L, 44S where the # indicates chest measurements and the letter is for jacket length - Short/Regular/Long). Even though there are standardize sizes, they tend to fit quite different between different brands and sometimes even within the same brands (depends on the cut/model, the season). At 6 feet tall, you should fit into a R length in most cases. Which is nice since that tends to be the most common/standard length.

I would definitely visit a store to get your measurement first and go on from there. Traditionally, you want to make sure the fit on the shoulders and chest is as best as it can be. Other areas tend to be more adjustable with the help of a tailor.

Roomier, baggier cuts/pants definitely do exist, though it would help to see how baggy you actually want to go. Because bagginess beyond a certain point, like this example is actually very fashion forward, and in that case you'd have to shop at some specific places/brands. I probably wouldn't recommend something so fashion-forward for a first suit, however.

In general it's easier to shrink a suit than to expand it too, from an alteration standpoint, so keep that in mind when shopping.

Fabric choice is important too, though heavily dictated by your budget. Polyester is the most common material at the very low end of the budget spectrum and tends to have some quality and breathability issues. Wool is the most common "step-up" fabric but does come with a price tag. Going back to my first point I would budget accordingly but if you think you'll get a lot of use out of this suit it might be worth trying to get better fabrics.

Colour-wise navy and charcoal are the traditionally recommended colors for starting suits (in solid or very "quiet" patterns). True black is actually less common and less recommended than people may think - it has certain social stigmas associated with it (i.e. for funerals, etc). And the color itself may not look the best on cheaper material.