In your TAI options (the minimap to the bottom left, there's a round button for options), select 'round numbers on torpedo track'. Align the matching numbers and fire.
Or.. lock the target in the periscope view. Keep your 'crosshair' in the centre of the target. Unlock the target and the full information will appear in TDWs UI dials - speed, AOB, everything (except identification) - and then fire. The last will hit almost every time unless the target evades, same for the first - it just requires a bit more time to align the target properly.
Either of these should help in the interim while you learn the more... complicated methods.
