I kinda hate to make my first post here by calling someone out, but there are some rather big misconceptions/overgeneralizations here so far. So my apologies in advance if I offend someone
1) "How to dress" is equivalent of asking "How to get laid", there's no single, simple answer without resorting to oversimplifications. And saying that it's only about fit and colors is pretty much like saying that all there is to body language is standing straight.
2) Suit? In most cases No, unless you really know how to rock one (then it could work just superb), but if you knew how to do that you probably wouldn't be asking this question
Dress shirt and jeans? Sure, this is what most guys resort to: dress shirt/shoes, jeans and a coat/jacket - its quite a safe choice and hard to completely mess up. On other hand, safe means you're really not utilizing the potential of looking your best.
One more side note: you can build outstanding outfit from the components mentioned above, but this doesnt mean that any pieces will do the work.
This is a huge topic so for the moment I'll just note main points:
-Dressing sexy is very different from dressing well and lead to very different results. And even though dressing well will not women attracted to you, dressing sexy definetely gives you an extra edge before any game even starts.
In other words this means that before you do anything, based on your style (body language+dressing+grooming+physique) you are already falling into one of the categories: "not attractive", "attractive for long-term relationships", "attractive for sexual encounters & short-term dating" (more on this in the next point). Sure you can move from one category to another with "game", but why play against the odds in the first place?
-Knowing your audience (what kind of girls you're into and how they dress) is great and does add an advantage, however, if you look at the research there some universal rules that majority of women find attractive for sexual encounters and short-term dating. There are also universal rules that women find attractive for long-term relationships. This means you could be "in her crowd" by dressing like she does, but if you don't tick the right visual triggers that make you attractive, you fall into the "un-attractive" category.
Note: here we are talking purely about physical/visual features that women find attractive.
Okay, now you might wonder what this has to do with dressing?
Well, we can't change our height and changing physiques takes time (though a worthy investment imho), so clothes is one of the few things we can control in the short-term that will affect how we are visually perceived:
F.e. In a poll more than 80% of women reported that for both short term and long term relationships they prefer man that is tall. (reasoning: being 6ft or taller just screams dominance)
Now you can't really do much if you're actually shorter, but you can build monochromatic outfits that will make you LOOK taller and make sure to wear jackets with three or less buttons. Furthermore wearing a v-neck with a necklase will help draw attention from your height to your shoulder/chest area.
In another poll the ideal physique for men has been very one-sidedly reported to something similar of the one B. Pitt rocked in "Fight club" (and body builder physique got least votes, so be careful with looking too big).
What this means is that while we work our asses of in the gym to actually build this physique we can use simple approaches to visually get as close as possible to this "ideal physique" to trigger the attraction and before we even say anything, drop in the right category we want to be in.
Now, these are just the basics on going in detail would fill at least a book
So let me just finish with this, dressing sexy is about creating a style that is both comfortable for you, emphasizes your best features and camoflages the less awesome ones and just as importantly, triggers the right evolutionary mechanisms for women to find you visually attractive.
Now, even though I can't give you specific advice without actual details, feel free to compose a more detailed question and I may be of assistance.