Your visibility and sound -the two things emanating from you that alert guards- all vary based on your stance, movement speed, light, weather, camouflage, proximity to the enemy, the direction they're facing, their level of suspicion, and so on. Your abilities all need to be experimented with and put into practice, but when you do, it totally pays off.
The best things you can do before you unlock better gear are to take your time marking every available soldier from afar before infiltration, exploit crawling if you have no choice but to move in front of the enemy, and the use of cover to look around. Keep your distance from the enemy and above all be patient. Generally speaking, the slower, lower, and further you are from them, the better. Don't take out soldiers if you don't have a good way of getting rid of them without them being missed. Every single action you do aside from simple sneaking has inevitable repercussions if you take too long. Keep an escape plan in mind. Set up diversions if you need to.