To get softer notes, use less pressure and/or lower bow speed. Louder notes, the opposite - more speed and/or more pressure. Tilting the bow so that there's less hair in contact with it reduces pressure, as well as putting less pressure on to begin with. You can also exert negative pressure (on the string) with your pinky.

There's a complication to that, though. Which soundpoint you use (where on the string the bow hair contacts the string) makes a big difference in how fast or slow, and how light or heavy you can play.

The closer to the bridge you get, the more you want to use more pressure and slower speed.  As you move out towards the fingerboard, the speed goes up, and the pressure goes down.

By convention, there are 5 soundpoints in that gap, each about the width of the bow hair. #1 is the one closest to the bridge, #5 is the one just short of the fingerboard. You can, of course, play anywhere on the string, but to make it easy to talk about, that's the convention that's used.

#1 high pressure, slow speed

#2 medium high pressure, medium speed

#3 medium pressure, medium speed

#4 light pressure (frequently just the weight of the bow), medium high speed

#5 very light pressure (you may have to use some negative pressure to make it light enough), high speed.

#3 and #4 (or somewhere between them) are the easy ones for most people. Exactly where in there you choose to play depends on what is most natural for you. If you're a bit heavy-handed, you'll probably want to move towards the bridge. If you naturally have a light touch, and speed comes easy, you'll move more towards the fingerboard.

For each soundpoint, there's a minimum pressure and a minimum speed that you can play at and still get a clean note. There's also a maximum pressure and speed. Pressure and speed interact with each other, so the minimum pressure may require something faster than the minimum speed and vice versa. (And the same for the maximums, of course.)

I'd suggest picking a soundpoint (just observe where you play now when the notes are sounding good), and start experimenting with more and less pressure, and more and less speed, until you find out where the "edges" are (for that soundpoint). If you're playing with as little pressure and the slowest speed that soundpoint can handle (and still give good notes), that's your ultimate "soft" sound (for that soundpoint). Same reasoning with the maximums.

It's good to be able to recognize when things don't work.  Use the first position (about a bow's width from the bridge, with the bow tilted a bit so most of the pressure is on the far side of the bow hair, and right next to the frog), and pull the bow very slowly, with a lot of pressure.  You should hear a fairly low pitched sound that I describe as crunching.

Now move the bow back and forth across that same point, with no more pressure than the bow weight. Use from the middle of the bow to the tip. Move the bow as fast as you can while still having good control. You should hear a high-pitched whistling sound.

The crunch means you need more speed or less pressure (or both). The whistle means you need less speed, or more pressure, or both. There's one last gotcha. If you dig in hard out around soundpoints 4 and 5, you'll hear the pitch go down. You're stretching the string so much it's gotten a little longer. That's too much pressure, whether you hear a crunch or not. If you want to use that much pressure, you're going to have to move your soundpoint back towards the bridge.

Now, remembering all this stuff when you're actually playing a song is your problem.