Start at the bottom, work your way out. Assuming you're not going for a workout and instead hanging with your friends dining and drinking, or other casual night out, here goes...
Go to any dept store that carries major brands and buy yourself 10-12 pairs of black (or white/grey if you prefer) Calvin Klein boxer briefs. These can be identified as fitting nicely but not too snug with two buttons in the front.
While you are there in the underwear section pick up a half a dozen Calvin or other decent brand of black and white tshirts. Crew neck,
not V neck. Whatever brand you go with should be thick enough so you can't see through them with some good length so they don't shrink too short.
Pick up a dozen pairs of black, or any other dark colour socks, again from Calvin, Boss, or other name brand. Make sure they are at least 80% cotton.
Next we go for jeans. I prefer Diesel. Mid height waist (I hate low cut) will exclude about half the styles right out of the gate. Straight leg, in faded mid and dark denim. I have about 5-6 pairs in various styles.
Pants (trousers if you're English)... Hugo Boss make some fantastic casual pants, and so do the likes of Quicksilver. Look for a decent pair of oversized cargos, I have a Quicksilver pair in olive. Look for another denim like pair of pants in tan or some other interesting colour. I have a light rusty brown pair from Hugo Boss that have a faint pinstripe in them with orange stitching and trim that are dope.
Shorts, same as above. I usually go for Quicksilver or similar (I could be a spokesperson).
Shirts. I prefer full button collar shirts from the various surf companies, Quicksilver, Billabong, etc. They fit well, wear well, wash well, maintain their shape, and come in a wide variety of styles from plain to plaid to heavily decorated and embroidered. I have about 20 or so but I wear maybe 8-10 of those regularly. Get some in long sleeve and some in short. You can leave the cuffs on the long sleeves undone and/or roll them up if you like.
For a more dressy look get a couple crisp white Oxford dress shirts. You can also go for something like Ted Baker or Ben Sherman (I'm sure there's a million brands in the US I don't know) for a dressy casual shirt that is nicer than that from the surf companies. Again, these can have some fancy patterns, stripes and stitching to make things interesting. Think about grabbing a couple shirts that require cufflinks.
Shoes... there's a fantastic store in London called Size that sells all kinds of exclusive trainers from every major brand that you just do not see in the regular stores. You need to find a store like this in your area (where is that?) and buy 3 or 4 pairs of whatever you think looks great. I have about 6 pairs of shoes from tan, olive and bright orange New Balance limited editions (exclusive to Size) to black and purple and cream Adidas. You'll want to get some dressy casual white trainers too. I have a couple pairs. As soon as they get the slightest bit dirty chuck them.
size? - Buy rare & exclusive trainers, deadstock, buy UK, adidas, Nike, Puma, New Balance at Size Online
You should get a pair of Hugo Boss or other black dress shoes, plain classic wingtip style and very sharp looking.
You could also get a pair of other more casual leather shoes in black or brown. These could be boots, brogues, blunted toes or rounded... whatever you like really. Wear your favorite jeans and go shopping.
You need a black belt and a brown one if you get brown shoes. Wide and plain.
Jackets. You should have a couple casual collared jackets from the likes of Carhartt, Billabong, Quicksilver, or whatever. Straight cut, open cuffs (no elastic crap) zip up with a couple pockets.
http://www.carhartt.com
You should have a dress coat, mid length in cashmere, from Armani or similar. Spare no expense.
I think that's about it. Finish with an IWC and your favorite bling if you care about such things.
Good luck.