I recently switched from focusing on learning rhythm guitar to lead and I'm having some problems. When I try to play a melody for example Metallica's 'Nothing Else Matters' or Led Zeppelin's 'Stairway to Heaven', issues arise as to which finger on my fretting hand should go where? Because of this my playing is very slow, with poor timing and just sounds bad on the whole. While I play I have to decide where to place my fingers and I've noticed changing notes/frets by each individual finger has become very slow. Is this something that I have to work out by trial and error to find what is comfortable? Help please, any advice that you guys have will be greatly welcomed and appreciated. Thank you in advance.
It's something that will take care of itself as you progress. You'll be easily able to use the correct fingerings without even thinking of it if you keep practicing. Also, learning the solos / melodies in short sequences helps with your current situation, you'll just have to learn them so well you don't have to think about it.

Guitars: Ibanez SV5470F, Ibanez Xpt700, Fender MIM Standard Stratocaster ('04-'05), Jackson Ps-2
Ashton AG200,
Amps: ENGL E530, Bugera 6262-212,
FX: TC Electronics G-major 2, Behringer EQ700, Morley Volume / Wah
Try to group a few notes into chord shapes whenever possible. This should identify which fingering to use. If this isn't possible, then doing whatever feels good is the bet way to approach it.
"If I could prove by logic that you would die in five minutes, I should be sorry you were going to die, but my sorrow would be very much mitigated by pleasure in the proof."
G.H. Hardy
just go for what feels comfy, with enough practice you wont need to think about it but some songs just have tricky fingering

good luck with it
Are you talking about the fingering? Here's a quick guide for what fingers go with what frets in different instances. Using the 5th fret as the root.

Frets:         5, 6, 8
Fingers:       1, 2, 4
Frets:         5, 7, 8
Fingers:       1, 3, 4
Frets:         5, 7, 9
Fingers:       1, 2, 4
Frets:         5, 6, 9
Fingers:       1, 2, 4
Frets:         5, 8, 9
Fingers:       1, 3, 4
I don't know , i never really had trouble trying to figure out the fingering. You just have to look ahead. You might think that hey this fingering works awesome for this bar, but then realize that it's a bitch to get out of to another bar and probably another fingering. Look for something that requires the least movement going in between one part to another and that it's comfortable.