The best way to learn is to experiment. Take scraps and solder
various shaped pieces on to each other. Watch when, how and where the
solder flows in relation to the shape and thickness of the piece in
relation to the placement of your heat source. Use the various temps
of solder and see what they do. Try various metals, each has their
own "personality" A good exercise for torch control is to solder
balls of solder together without melting the ball shape, like making
a tall snowman. Basically just know that
Solder won't fill a gap, so file your pieces to an exact fit, hold
it up to a light and see if any light shines through.
Solder won't flow across dirt, make sure the pieces are free of
any dirt, oils and oxidation then apply flux.
Solder flows toward the greatest heat, so position solder on one
side of the piece and draw it through with the torch. You want the
two pieces at the join to heat evenly, you will see that the larger
part will need more heat than the smaller or thinner part.
That should get you started.
Have fun, Marta