You said you just started 3d modeling and really, it's not bad.
If you want to learn texturing you have to learn unwrapping the uv's first. As you go, you will learn that you have to keep your uv's in mind when you are modeling. You don't need to use boleans for this model, it will make the unwrapping process needlessly harder.
This model should consist of multiple elements. The big box which is the largest part of the speaker, the front panel, the 2 black cylinders, the volume button and the tone wheel. The headphone and microphone input can be textured, it's going to cost way too many polygons for something that small.
Now you can start unwrapping. It can be a long process depending on the complexity of the model. So understand that it doesn't matting if it's taking a long time, especially when you are just starting out.
Apply a checker material to your model so you can see if everything has the same resolution and where it stretches out. (They should all be squares and the same size.)
You can't use the picture you are using as a texture because it's stretched and has lighting information on it in the top left. It's not really how the texturing process goes anyway.
Start by unwrapping every element one by one.
Try to fit all the uv islands in there using as much space as you can. It doesn't even matter where you put them.
Once this is done you can start texturing in photoshop, substance painter or whatever you use. You are going to need multiple maps. I would start with the diffuse map, which are the base colors.
Later you can add a normal map, roughness, metallic, ambient occlusion, maybe even emissive for the led lights and so on. Try learning about all of these.
Once you did all of this you can also learn about baking, which should be done after unwrapping and before texturing. You can bake good looking shadows on a ambient occlusion map or maybe bake some nice detail from a high poly version of the model into a normal map.