With SAI you're ok, its easy to use and there are tons of tuts around, photoshop is way better, but since you're
starting SAI works pretty well.
First of all, try getting used to your tablet, don't rush to draw cuz you might get frustrated. Go around your desktop
to get used to the ratio of the tablet, it might be difficult at first, then open SAI and draw lines, circles, any kind
of shapes, so you can get used to its pressure and how to make clean strokes. Also if your tablet have shortcut buttons,
use them! its much quicker to work with the tablet shortcut buttons, but that is, if the tab have them. Make sure to
look for a starter guide for SAI, a video or whatevs that explain the tools and the basics of the program.
Once you catch how to move around and how the pen works, try drawing simple stuff like clouds, flowers, trees, simple
landscapes, look for the different brushes and settings (there are lots of settings out there, just look for them, also
there are many free brushes you can download), don't be afraid of making mistakes, you're practicing after all.
You can even grab a simple illustration and trace over it, to learn to make linearts. Remember to use short and fast
strokes over long and slow ones, the faster you do them, the less stiff your hand will be and you'll get sloppy
As for coloring, remember to use layers, separated colors and lines are better to work. Start simple, still life could
be a good start as mentioned above, look for blending color techniques and basic painting of lights and shadow.
And finally, ctrl+z is your friend, do a line, if you're not happy with it, erase it and try again and so on until you
got a line that looks great, it might get time, but the results are worth it. Don't give up and practice all you can. I
know I mention "look for tuts" a lot here, but they are tons of help, so again, look for tuts and references,
they're out there for you to use them.