Is this aluminum channel, or is it steel? Will a magnet stick to it? This is important to know, since that would influence my choice of tools. A magnet will stick to steel. (I'm pretty confident this is not stainless steel from the looks of it, but I might be wrong. If so, then a magnet will not stick to some grades of stainless. And stainless offers some difficulties of its own for a novice to cut.) This looks like a steel doorframe channel.
For example, on steel, I'd go with an angle grinder as the fastest way to make the cuts, and a way to make a fairly straight cut. But aluminum would gum up the cutoff wheel.
For aluminum, I'd choose an oscillating multitool, like that from Porter Cable, Dremel, Fein, Bosch, etc. These tools take a toothed blade that oscillates rapidly, and will cut through aluminum easily. Again, you will get a fairly straight cut with the proper choice of cutting blade.
Lacking either of these tools, or being unwilling to buy one, I'd go with a hacksaw, which can cut either metal. Of course, a hacksaw will yield a somewhat jagged cut unless you have some practice here.
One problem with the cut will be in minimizing any vibrations of the metal as it is being cut. A jigsaw, or oscillating multitool, or hacksaw will all cause the material to bounce around. A good vise will help solve this. In fact, my preference would be to clamp a piece of sacrificial wood inside the channel, then put it all in a vise. That will yield the best and most vibration free (therefore cleanest) cut possible.
In any event, I'd expect to need to clean up the edges, rounding them so there are no sharp edges. A file will do for a start, then I'd switch to some emory paper. (Ok, sandpaper in a pinch.)
If the material is steel, make sure you paint the surface afterwards with a rust inhibitor, as the cut edges will rust otherwise.