Own bearing is your heading, i.e. the direction you're travelling.
Relative bearing is the position of the target relative to your sub. You get it directly from periscope/UZO/Binos.
TGT in front of you : 0° behind : 180° left : 270° right : 90°
As a help you find these on the inner wheel of the compass and on the bearing overlay at high zoom levels.
Target course is the heading of the target (the direction it travels). You get it by plotting on your map several positions and drawing a line through these successive positions.
If you want to have the speed as well, you plot these positions every 3 mn 15". The distance between 2 plots give the speed. For instance 0.5 km = 5 Knots, 0.6 km = 6 knots and so on... If you use "map contact updates" these will be precise. If not, take more plots to average errors in measuring.
As said above, rather than calculating you can use the protractor tool to read the angle directly.
Hope this helps
