I understand weather can change unexpectedly, and local sky condition (e.g. cloud types) can't be predicted, but if I want to fly glider at a specific location, say, some time next week, and use weather forecast to pick the best possible day, what should I look for? e.g. "Cloudy" vs. "partly cloudy" vs. "overcast" vs. "sunny", is one preferable to another? (I wish the forecast would say something like "Cumulus clouds abound")
For thermal soaring, which is the majority of the time in the sport in North America, the best soaring days are usually the day or two or three after the passage of a cold front with high pressure behind it, where light to moderate winds are forecast. You want relatively unstable air without any inversions, no mid to high level clouds, and low humidity but preferably enough to form cumulus clouds to mark the thermal tops (if the air is really dry you have no clouds at all and have to troll randomly looking for lift, or look for thermaling birds, or look for other gliders that have found lift). In other words, it's those clear sunny days after a high moves through, the temperature cools off, and you get those nice puffy widely separated cumulus clouds and light to moderate winds.
If a front ahead of high pressure has a strong pressure gradient behind it, it's likely that the winds the next day will be strong and this is bad because it tends to break up the thermals. In that case I would plan to go the day or two after as the high moves in and the winds die off.
There are a lot of other factors, like the ground moisture content (wet ground sinks heat and inhibits thermal formation), but as a way to try to judge the best likelihood of good soaring conditions, the 2-3 days following passage of a cold front rule is the most reliable.
If you're not much at interpreting weather maps and can only go by the public forecast, and have to pick a day in advance, try to pick one of the first days following the day with storms and a temperature drop in the forecast due to a cold front, where it shows sunny or "partly cloudy" (which will usually be be desirable convective cumulus) for a few days after, and avoid really windy days which if it happens is typically the first day after frontal passage.
Exceptions to this are desert areas which have sunshine and good convective conditions for thermals pretty much all the time, and places like Florida where during any kind of sunny weather there is always convection (there is a localized thunderstorm somewhere in the state just about every day in summer).
It very much depends upon what type of flying you normally do. I can only really comment on what I do, you will have to adjust for your own circumstances. At my local glider field we mostly fly on ridge lift as we have a very wedge-shaped ridge right next to the airfield, it has a very steep southwest face that tapers away towards the North. We also have a small range of hills to the SW of the field that faces NW that gives ridge lift. This means that I predominantly look for wind direction and strength, wind from the West gives good lift but from the East the wind just makes massive sink. We had a wind from the East for a couple month earlier this year and so all we managed was circuits.
The second most important lift for me is wave. I live in Scotland is quite small and which has mountains all down the western side of the country. I keep an eye out for wind coming off the sea and striking the mountains as this causes wave which ripples all across the country.
Cloud is obvious. I look for a decent cloudbase, high enough to allow me to fly without entering the cloud. Also I check for rain forecast and avoid rainy days. Cloud cover is less of an issue to me as I live in a hilly country where thermalling is less important than hill lift as there is always a hill nearby.
The best advice I can give is to talk to your chief flying instructor at the club and get advice specific to your airfield. I'm guessing that from the question you are a beginner flyer and so your flights will likely be around your airfield. Your CFI will know the area intimately and can tell you just what to look for and what websites to go to for forecasts and how to interpret them.