Welcome silve. You surely picked about the hardest sky you could, right down to the reflection! Are you allowed to use the gradient tool or only brushes?
If gradient tool, I would make a gradient, go to new layer and start brushing, soft low opacity brush to make subtle color changes, and keep layering like that. You can experiment with blending modes like overlay, vivid light, hard light.I would do pretty much the same thing with just the brush. Make three, maybe 4 bands of color, then do the layers.
To do the sun reflecting and get it to look natural is a bit tougher. I'm not sure I understand the limitations of your use of Photoshop tools or whether you are just allowed the default brushes (I imagine so). Anyway for the sun, you can make your circle, use the marquis tool to cut it in half and make new layer via cut. You'll want to soften the base of the sun as you would see in a sunset. For the reflection, if you can't use a displacement map, then use erasers and brushes to make it look like it's laying on the ripples of the lake. Depending on the effect you want, if you have a fancy cloudy sky around the sun, you may just want to transform, flip vertical, then use brushes and erasers to alter the reflection. Play around with opacities and blend modes on the layer till you get the look you want.
I hope this helps, long-winded as it was. If you want more specific stuff, please come back. Oh and do share the work you do. We have challenges sometimes for people to learn new techniques and show off! A recent one was drawing with brushes only.