How Often Should I Receive the Sacrament of Penance?
The Church requires that each individual at least receive the Sacrament of Penance once a year, prior to Holy Week, if at no other time.  The typical recommendation for the good of souls is to approach the confessional once a month.  Some go every week to receive the grace of the sacrament.  

Should I Only Go if I Am Aware of Being Guilty of Mortal Sin?
One can go to confession for venial sins if they so desire.  If one is in a state of mortal sin, receiving the Sacrament of Penance is required before approaching again for Holy Communion.  One should not wait until Easter to confess a mortal sin.  The soul that dies with unabsolved mortal sin cannot enter Heaven.

I Forgot to Perform My Assigned Penance.  Is My Sin Still Forgiven?
Yes.  Penance has no effect on the absolution that takes place during the sacrament. Doing your assigned penance is highly meritorious and helps mitigate temporal punishment (i.e., purgatory) for those sins.  But once you are absolved, you cannot become un-absolved.  

Can a Confession Be Invalid?
Yes.  Although the priest cannot know the exact state of your soul, if an individual were aware of a particular grave (read: mortal) sin, and purposely withheld it from Confession, for example, out of concern for human respect or embarrassment, absolution would be invalid.  There are other scenarios, such as consciously withholding intent to amend one's life, that could invalidate absolution, as well.  

On the other hand, if you simply forgot to confess any sin which occurred at any time in your life, that sin is covered by absolution, and therefore, you have validly received the sacrament.

I Haven't Been to Confession in Decades.   Where Do I Even Begin?
The priest has one purpose: to heal your soul by the power of the Holy Spirit, acting in the person of Christ.  There is nothing to fear, because, frankly, priests have heard everything under the sun, and are not there to shame you, but rejoice in your return to a life of grace.  You may find it more helpful to make an appointment and meet with a priest who can walk you through the process.  However, if you prefer to remain anonymous, you can still approach Confession during its normal hours to receive absolution.  It's important, though to let the priest know how long it's been so he can properly pastor to your individual needs in this circumstance, and allay any fears or anxiety.   

Can Non-Catholics Go to Confession?
Technically, anyone can walk into a confessional, however, a non-Catholic can only receive "forgiveness", but not "absolution".   Absolution remits the punishment due for sin. It is the point at which God literally "put(s) all my sins behind his back" (Isaiah 37:18) and sanctifying grace returns to the soul.  Priests alone have the power to loose (or hold) sins (Matthew 18:18), and only a Catholic initiated in the sacrament can receive the effects of absolution. 

"Forgiveness" can be provided by anyone, but does not repair the soul.  

So What's the Formula of Absolution? 
Absolution may or may not be preceded by a prayer said by the priest, but the form of the sacrament requires at least the following to be uttered:  "...I absolve you of your sins".  

If the priest does not say exactly "...I absolve you of your sins," you have not received the sacrament.  It is not valid for a priest to say, "I forgive you of your sins," or "Your sins are forgiven," or "We absolve you," or "The Church absolves you."  It must be, "I ABSOLVE YOU."