Hmm I'm not a fan of sneak tbh I've tried a character but enemies seem to always notice me even with a relatively high sneak skill. Is that just me?
It's all in the technique. First off, always get Sneak to 100 as quick as you can. I'm not sure of the exact mechanics behind how Sneak works, but if it's like most games, it's a dice roll in the background, which determines whether you get detected or not in any situation- and the higher your level of Sneak, the more favorable your detection rolls are going to be.
Always get Silent Running perk at level 12- my favorite perk in the whole game, and any Sneak specialist's favorite. Before you get SR, Sneaking is a pain in the butt, because you can only effectively do it up close while in 'walk' mode, making it slower than hell, and quite dangerous. Once you have SR and 80+ Sneak skill, that's when it starts to shine.
Gear- light armor or clothing, at least until you have maxed out Sneak and gotten Silent Running- then you can get fairly close in medium armors, if you feel you just have to wear them. But stick with light armors for best sneak results. If you have companions along, give them light armor too. Not sure if your companion's gear factors into them being detected or not, but I choose to err on the side of caution.
If you want to do your best sneaking, play at night. Night's darkness, and the extra dark shadows available, vastly improve your sneakiness. Trying to sneak up close in daytime just gets you killed.
Stay in shadows and behind covering objects as much as possible, and always try to stay within the 180-degree arc which is 'behind' the enemy you're sneaking up on. When that enemy turns in your direction, freeze and don't move at all until they change direction and you are once again in their rearwards arc. Your enemy's sneak-detection rolls are much higher when you are in 'front' of them. Or if you have to move, move behind a cover object where they can't see you, if one is close by. Plan ahead- take note of all the possible shadowy places and objects which will block or obstruct your opponent's view of you as you move closer. Then use them.
Only get as close as you need to- unless you're trying to pick their pocket or do a melee/unarmed sneak attack, you don't need to be right on top of them to get an easy kill with most weapons attacks. Pop open a VATS screen every so often as you sneak towards them- once you start getting a 95% chance for headshots (or whatever body part you want to hit), you're close enough.
Be patient, always. Snipers and assassins don't rush in- they are silent and patient and take as long as it takes to do it right. If you aren't patient, don't bother with Sneak- go get a minigun. Since you will of course be in crouched sneak mode during your movement towards the target (you do realize the Sneak skill does not help you at all unless you are crouched, right?), you should see your stealth indicator at the top of the screen- pay very close attention to it. When it says 'hidden', your enemy is completely unaware you are around. When it says 'CAUTION', it means one or more enemies have become aware *something* is nearby- but not exactly where, and they don't have a target lock on your position yet. If you have companions with you, this caution flag will probably make those idiots freak out and go Rambo on you, attacking instantly and screwing up the whole show. Better off to leave your companions somewhere far away, if you are really serious about your sneakery.
But, if you wisely left your companions elsewhere, and the CAUTION flag goes up, do not panic- it doesn't mean they will actually find you, or start attacking. Sometimes, if you just remain totally still and crouched, they will lose the scent and go back to their normal business, and your indicator will go back to 'Hidden'. Other times, if they are moving too close to you when looking around for you in Caution mode, you can quietly slip behind a rock, a corner of a building, below a ridgeline, or into a deeper shadow... and also elude detection that way until they give up interest. With practice, you can raise the Caution flag repeatedly during your stalk of a target, and still get in for a silent Sneak kill. Be aware that some flavors of enemies have a lot better stealth detection than others, and can be very tough to sneak close to- Deathclaws are one notable example. Not impossible, but tougher. NIghtstalkers are a beach, too.
Another thing to remember, when trying to sneak-kill multiple enemies- if you use silenced weapons, you can kill enemies that are fairly close to others without alerting those others, as long as the one(s) you aren't attacking are facing away from you and your target, and as long as you get an instant kill so they can't warn their friends. It also helps to use cover for your attack in a way that you can only be seen by your target, and not his friends. Killing a target this way might put his friends into CAUTION mode, be if they can't see you due to your positioning, you have a chance to elude them and stay in stealth. You might be able to quickly and silently retreat to a safer spot nearby, until they give up and stop looking.
Now go forth and Sneak!