Amount of energy depends entirely on what deck you are running. Gyarados, a meta-deck right now, needs no energy to attack and only runs 'tech' energy like rescue and warp, while a less popular meta-deck called mewperior can have more than half the deck as energy. Generally, you want approximately half of your deck to be trainers and supporters, then just experiment with the number of pokemon vs. energy. Play a few matches (even if it's solitaire, it doesn't matter), and if you always draw too much energy then lower the count, and vice versa for pokemon.
I'm not sure how rotation works but I can't seem the point in all the deck building when a new generation is just around the corner.
Well, deck-building now gives you experience for the future. That and rotation is usually only done four sets at a time, and there are six sets still legal from Diamond&Pearl and Platinum in addition to the 4 HGSS sets and Call of Legends, so if you do the math, they probably aren't going to be rotating the HGSS sets, which don't really have too many cards relevant in the metagame right now, but are still worth a look. Then there's also the collection element of being able to say you have certain cards even if they're being rotated out of tournament play.
No, there has never been a totally clean slate rotation in pokemon, some sets that are legal now will be rotated while the ones released more recently will remain in format. They want sets to be in rotation for at least a year, sometimes up to two years. There wouldn't be any point to organized play if they scrapped a set they just released that year because the next set was coming out, if that were the case there would be sets released that never even saw tournament play at worlds etc.