I personally find using an “Examination of Conscience” guide book or leaflet to be really helpful when preparing for confession. I have mentioned this in other posts here on the forum.
Some parishes have the printed booklets or leaflets available at the time of confession. You can also get a booklet through your local Catholic book store.
When reading through a booklet or leaflet, it helps jog my memory to see if I might have forgotten anything that I had intended to confess, but that just slipped my mind.
As to your questions, yes, we are obligated to confess mortal sins. You should go to confession whenever you have committed a mortal sin, and need to confess it.
As to how often you go otherwise–once a week, etc–that would be up to you, and what you think you needed to confess.
The church encourages us to go to confession during Lent.
As for venial sins, I confess them. I think about what it says in the Catechism (#1862/63), and what I also read before too, and that is that occassion of venial sin can lead to mortal sin.