Short answer :
find altimter setting and use this calculator
Pressure Altitude = the altitude you get when you put in 29.92 in your Kollsman window of your altimeter. (this picture has 29.87)
This number and temperature is needed to calculate Density Altitude and will allow you to use your POH performance charts when preflighting.
First call your airport's ATIS/AWOS ,often there is a phone number , that you can call and hear the weather. Or look up the information on whatever source you use for your weather briefings. You will need the Altimeter setting or barometric pressure and the temperature for the Density Alitude (next step).
For example I just called the Hollister Airport and heard that the altimeter setting is 29.93 and temperature is 24. Standard pressure is 29.92 and temperature is 15C and used for charts we will need to adjust to reflect our flight in our conditions today. Since the numbers we have have at Hollister are different we need to correct for those non-standard numbers to figure out Pressure Alitude. If you are home and can't just dial in 29.93 to find your Pressure Alitude a rule of thumb says you + 95' to the field MSL for every .1" of mecury below 29.92 and -95' if above.
Hollister is at 230MSL - 9'(since it is only .01" off) = roughly 221' PA which is great but we are not done we need to use Pressure Alitude now and correct for non standard temperature which today is 24 not 15C. You can use either your E6-B to figure out Density Altitude, some POHs have the charts with temperature corrections or of course the internet has many calcultors here is one http://www.pilotfriend.com/calcs/calculators/density.htm
Using my Hollister example I see that the Density altitude will be that of 1498' much higher than the MSL of 230'. I would give myself a cushion of getting 30% over that figure. In othewords 1498' + 30% = 1948ish' .What does that mean?
The plane will be operating at an altitude of 1498' (1948') on the ground and will have less performance than if it was cooler and less humid. At basically 2000' I will have a much longer takeoff roll, less climb performance, less options if I have an engine failure between takeoff and landing.
High temp+high humidity = less performance
Is the runway long enough to take off? Will you clear any obstacles? If the runway or obstacle is anywhere close to that number don't take off wait till conditions improve
AOPA has amazing Free Online courses, their Mountain flying course Chapter 3 speaks to this, below is a little video to see the effects of Density Altitude. To see the whole course go to http://www.aopa.org/asf/online_courses/