First of all, welcome aboard!
A three mile-long lake is fine for sailing; the half-mile wide part may be a bit challenging though .
If the wind is below 5 knots (or so) you're probably not going to be able to get the boat to move effectively. Plan accordingly.
Whether going upwind or down wind, keep the outhaul loose and the jib cars forward. You want both of your sails to be "baggy" in light wind. You won't be able to "point" (go upwind) as well, but at least you will "go".
Macs are difficult to "tack" (but easy to "gybe") in light air. It's a skill you will gain with time but at first don't worry about using the engine to help with the tack. At some point you will want to raise your outboard out of the water. Most Mac power sailers (X or M) will lose about 1/2 knot with the outboard down.
You will need about 2 knots of forward speed to execute a tack. If you don't have a way to measure your speed just remember that the rudders will begin to get a little noisy around 2 knots. If you don't hear the water flowing over the rudders a tack may be be difficult. It is not a coincidence that two knots of speed is about what you can expect out of 5 knots of wind (see above).
Turn
gently into the wind. Leave your main and the jib alone until the wind hits the
main from the back side (the jib will already be catching the wind on the "wrong side"). Quickly release the upwind jib sheet, center the rudders, and immediately pull in and position the downwind sheet. You probably won't need to reposition your main sheet; it will just shift to the other side on it's own. If you are concerned about the boom moving across too quickly grab the main sheet near the middle with one hand to slow the boom as it's moving across. This will act like a shock absorber and will work in any amount of wind you should expect to be in.
The boat will have turned too far downwind at this point so you will (again)
gently turn into the wind to bring the boat back to the desired course. If you skip the "gently" part in either turn you will probably be in irons (pointed into the wind) so just plan on using your engine to finish the tack. More wind = more forward speed = more forgiving of a too-hard turn.
Going down wind is easier because gybing in light wind is easier and more forgiving (or so I've been told). Where I live, going "downwind" is just a legend. It is never an actual option because the wind is always coming directly from wherever I want to go, it's not blowing at all, or it is a hurricane.
If you make a mistake while gybing you can usually pull (or push) the boom to move the main over to the "correct" side of the boat which will make moving the jib trivial. You can adjust the length of the main sheet as you gybe but using the "grab the main sheet in the middle" trick is usually easier when you gybe. Don't forget to duck.
Even in light wind
always have your ballast full. If even one sail is up the ballast is full; never empty, never partially filled.
You
can raise your centerboard most of the way when you are going directly down wind. It does not do much on that point of sail and you will go a bit faster. If you do so you
will forget to put it back down when it comes time to go upwind and it will take altogether too long to figure out why as you are blown sideways across the lake.
Again, welcome aboard!