When and how do you use the spells Specter and Ressurect?
Specter: Can only be played at the end of the Aftermath phase if your HP reaches 0.
Ressurect:Can only be, and must be, played when your HP reaches 0.
Also,if you play a card with single target, can you target yourself?
Not quite sure I understand your confusion, as how they are used is written on the card.
Resurrect: As soon as your HP reaches 0, you can throw down resurrect and convert any amount of your remaining EP for HP. You could die right after depending on how the aftermath plays out.
Specter: At the end of the aftermath phase you can play specter if you want to. You'll gain 4 HP. If you manage to inflict at least 3 DMG to a single for on the very next turn, you can stay alive. If you don't, your HP returns to 0 at the end of that turn.
You can't target yourself with a card that targets a single target. You CAN use those on teammates however, but it's rare that you would ever want to.