1) pentatonic is a safe bet almost every time. Careful, it gets boring fast if you use no imagination.
2) usually you play a scale from the key. Since you seem lost, I wouldn't recommend trying otherwise yet.
3) you can use any notes that your ear tells you that they are not horrible. It doesn't have to start and/or end on root note or any note in particular. If you hit a bad note you will know it immediately.

playing leads is not something you can figure out in a couple of days or weeks. it takes time to understand what you need to do to create good solos. you also really need to be able to hear and recognize the notes you are playing. this is not an automatic given for everyone. some ppl just don't hear that some notes sound the same. you have to develop your ear a bit. the thing to do is to sit down with some songs and figure them out WITHOUT the use of tab. just use your ear. learn as many songs as you can with the method. after a few months of this process, you start to automatically hear and RECOGNIZE all the keys. this is crucial in lead playing. when you are flying thru a solo, you want to be able to make quick corrections if you miss a note. if you can't even hear if a note is wrong or not, then it is REALLY hard to make your solos sound good. so developing your ear is really important for this application. and this won't happen overnight. you have to play by ear for a while to develop that skill.

now learning the major scale and the chords that surround it. will automatically teach you WHERE a lot of notes on the fretboard are. this will advance you at a MUCH QUICKER RATE. by learning the major scale as well as other scales, you will at least learn WHERE to put your fingers even if you don't necessarily hear the notes to pitch.

so a combination of learning as many songs by ear couple with learning the major scale and a few others CAN get you to where you want to be within..say..3 or 4 months with steady practice. just be patient. it can be a frustrating process, but it IS a necessary one. and of course during this time you also want to work on unison bends,hammer ons, taps, and as many different kind of bends that you can. try to enjoy the whole process. that's half the fun anyways.

another good thing to do is to try to find some other guitar players and jam with them as much as possible. just take turns churning out random ideas and solos. this will help you develop some better feel for the whole process. and it's a ton of fun too.....