(In progress... Will edit in more later)
Hey there. Before I get into this let me ask you how serious you are about drawing and painting. Is it something you want to do in the future or if it's something you just do for fun every now and again? You've stated you're okay with a mouse. If this is anything more than a very occasional hobby you're going to want to invest in a Wacom tablet. You can the Bamboo model (which is the cheapest) for about $90-100. It's decent enough if all you're doing is a hobby. I myself work on and Intuos 4 (9"x 12").
The first things you're going to need to know when drawing in photoshop:
Hotkeys will make your life SO much easier. Here's some useful I know:
B - Brush tool
U - Blur tool
I - Eyedropper tool
G - Paintbucket and Gradient tools
L - Lasso tool
Command/Cntrl +/- - Zoom in and out
Command+T - Free Transform
I highly recommend mapping a hotkey for flipping the canvas horizontally (I use Command+R. This allows you to easily check composition and will really help you determine what need correction and what needs finishing.
Layer modes:
Layer modes are one of your most important tools. Become familiar with them so that when adding layers you know exactly what you're doing. Here are some useful modes:
Normal - Everything appears as you paint, with the opacity you've set. Good for finalizing your image.
Multiply - Everything darkens the image below using the colour currently selected. Good for establishing shadows and outlines.
Overlay - Everything becomes lighter and transparent, good for adding texture and lightening up various things. Also good for changing the colour of certain things.
Color - This one has limited uses, but I often use this to apply gradients, with low opacity right at the beginning of my paintings. It allows me to establish a base colour palette.
Brush Modes:
Familiarize yourself with how various brushes work. You're going to mostly stick with the basic hard round. Familiarize yourself with:
Opacity - You can set opacity jitter to pen pressure on a tablet, which speeds up everything a lot. You will want to work in higher opacities for base coats and for highlights, using harder brushes and lower opacities and softer brushes for shadows.
Hardness - How clear or fuzzy the brush comes through on the image. Play around with hardness when exploring light and shadow. Especially dealing with texture.
Shape Dynamics - Familiarize yourself with minimum diameters. Once you get into more complex brushes, start experimenting with x and y axis jitter and the like.
Custom Brushes:
As you familiarize yourself with digital painting, the appeal of custom brushes increases. It gets to a point where you want to use them because, well, it seems a lot easier than just rendering it yourself. Be sparing with the custom brushes you make and use. I have a whole boatload of brushes and custom brushes. I try and only use about 5 or 6 per digital painting. Otherwise your doing less your own art and a bit too much stamp-making.
Blurring and Smudging:
These tools might seem great at first, especially the smudge tool. Use them a limited amount. The blur filters are a much better resource. But explore their uses anyway.
Lassoing and transforming:
When you need to alter shapes' size and orientation get familiar with the regular lasso tool and start experimenting with the free transform tool as well as the warp, skew and perspective tools. They all have their uses.
If you continue to post your work in the thread that Diagonal Horizontality linked you to, I will be able to give you specific advice on how to improve your drawing skills. You're doing okay right now and I'll help you any way I can.