What does the first Sloka Mean?

Any Pooja performed starts with this sloka.

"शुक्लाम्बराधाराम विष्णुम शशि वर्णं चतुर भुजं

प्रसन्ना वदनं ध्ययेथ सर्व विग्नोपाशान्तये ||"

"Shuklambara Dharam Vishnum Shashi Varnam Chatur Bhujam

Prasanna Vadhanam Dyayeth Sarva Vignopa Shantaye"

Meaning (to my knowledge)
In English
शुक्ल (shukla - White) अम्भार (ambhara - cloth) धरम (dharam - wearing)

शशि (shashi - moon) वर्णं (varnam - colour)
चतुर (chatur - Four) भुजं (bhujam - limbs/hands)
प्रसन्ना (prasanna - pleasant/happy) वधनम (Vadhanam - face)
द्यायेथ (dhyayeth - meditate) सर्व (sarva - every) विग्न (vigna - obstacle)
उपशंताये (upashantaye -will go away)
PS: Thanks to Samkritha Bharathi for opening a path to learn Sanskrit (www.samskritabharati.org/sbindia/)

On mediating the one who is attired in white cloth, who is of color of moon, who has four limbs and who has a pleasant face, all the obstacles you face will go away.

As i have been told by my elders at home, this sloka is a prayer to Lord Ganesha.  While uttering this sloka you also have to close your palm and knock your fist against your forehead.  I also have heard and read about some controversy of this sloka referring to Lord Vishnu.  The characteristics described goes well with him too.
Some reference give the meaning "All Prevalent" to word विष्णुम "Vishnum".

With Hinduism having plurality in god and openness for any individual to follow his path, the interpretation of which specific deighty is referred is upto ones interpretation.

I also happen to attend NLP classes done by Only Success (www.onlysuccess.net ).
I recall Praveen talking about state, its true you can't think about bad things and still smile.
Smile is very addictive. How many times have you not responded to a child's smile?  Whatever be your condition, that smile immediately brings you back into a pleasant state.

I strongly believe this sloka is meant to get one into the right state before one proceeds with the rest of the pooja/ceremony.  You recall that pleasant smile of Ganesha or Vishnu or even Child next doors, when uttering this sloka in bright white background to enhance the effect, you are all set to shatter all of your obstacles.

Happy Vinayaka Chaturthi.