In all my years of flying, I've yet to come across a paved runway not long enough to get a safely loaded C172 up and over the trees; even on the hottest of summer days. Granted, I've not done much mountain flying, where field elevation is a concern, but for the sake of sim flight-planning, at this time, we'll assume that all paved runways will be long enough... and we'll touch on density altitude, in its simplest form.
Density altitude is the first thing to address during your weather planning. More times than not, it will just be a formality, but if you ever get to flying heavier/faster airplanes, it's good that your first instinct is to consider density altitude. For sim-planning, all we need to worry about is the temperature. Hotter air is less dense, and that's a triple-edged sword to a normally-aspirated, piston airplane.
1) There is less air available for the wings to generate lift.
2) There is less air for the propeller to generate thrust.
3) There is less oxygen for the engine to generate horse-power.
The affect can be dramatic. On a hot, summer day, where the C172 is well past the departure end of a 5,000ft runway, almost ready to turn to the crosswind leg before reaching pattern altitude (normally 1000agl).. that same C172 can be at pattern altitude by the end of that 5,000ft runway, on a cold winter morning.
In short... air gets less dense as altitude increases. Hot air is already less dense and can make the plane "think" it's already at a higher altitude, before even taking off. Hence, density altitude is the actual altitude, adjusted for temperature (and atmospheric pressure and relative humidity, but we don't need to be that advanced in simming, yet). It's so important, that most AWOS/ASOS/ATIS broadcasts will include density altitude.
Since all our flights at this time will be VFR, all we'll account for in our planning is winds aloft... and that there isn't a fore-casted crosswind beyond our ability, at any of our stops, or alternate airports. Terminal Aerodrome Forecasts (TAF) are your best source for weather at specific airports. These (along with winds aloft) can be found at the weather planning site listed earlier in this thread.
I'm going to include the pertinent data for our flight... we can discuss how to navigate the weather site as we go.
A TAF map gives us a glimpse at the prevailing surface winds...

The little flag from each reporting station points to direction from which the wind is coming... and each line on the flag represents 10knots, Since there are no flags showing more than 15knots along our route, it's safe to assume that there will be no crosswinds with greater than 15knot components (the C172's published limit). This map also shows that there will be marginal, if not IFR weather, at times... but for now, we're just concerned about winds.
TAF data for a couple of stations along the route (green arrows) confirms things for us.
KTOL 151501Z 26011G
15KT 10SM