Such good replies! I think you all said most of what I would have said.
Let's see, is there anything different I can addd . . .
Well, first of all, as a beginner you are probably gripping the horse more than you need to , so you are already applying more leg pressure than is "nuetral". the lesson horse knows this and is good at tuning out the "grip of death" that newbies ride with. However, it will mean that to make an aid feel different, you have to make it, well, . . . different.
That mean that it must be either a stronger squeeze or have some kind of movement to it. I like the use of a bit of movement. So, when I put both legs on, like to ask the hrose to trot off from a walk, I kind of put them on in a way that feels a bit like "plumping a pillow" between my calves. So, I lightly bump the horse, :"bump, bump, bump" . This has a more enlivening action than a hard squeeze. Some horse dislike being squeezed between your calves and will suck back behind your leg. You'll learn to recognize that feeling over time.
But, you liven up your horse by livenning up yourself. So, along with putting a LIVE leg on it, you think "let's go!" and you kind of mentally add energy to your seat and mind. I know it sounds woo-woo, but it works!
Another way to put a leg aid on that will feel different is to "brush" your horse's side with your ankle. This feels very different from nuetral and should get a reaction from the hrose.
Getting a reaction, or as I say, "a change" is what it's all about. all aids are meant to signal that the horse make some sort of change. There is nothing that says you can't experiment a bit with puttting your leg on a bit more forward, backward , heel in, calf in, ankle bone on the side of horse, whatever ,. . .and just see what sort of "change" you get from the hrose. Trial and error.
Eventually, both you and he will remember what got the best change and you'll repeat that aid.
Horse's arent like a machine, that there is certain buttons that always get certain reactions. They are trained to have predictable reactions, but some work better in one way, others in another.
Riding looks so easy but is MUCH deeper than it appears on the surface.
When I first started Dressage, I aske my teacher, "How long does it take to become a good Dressage rider?"
"At least Ten years" was her answer!