Simultaneous Equations with way too many unknowns..

Yea, so equations aren't my bestfriends, and I'm now stuck with two simultaneous equations, and I just don't know where to start, so if someone could help me and show me what to do, I'd appreciate it.

'Find the values of *m* such that these equations have no solutions':

3x - my = 4

x + y = 12

'Find the values of *m* and *a* such that these equations have infinite solution sets':

4x - my = a

2x + y = 4

So if someone could patiently guide me through step by step, that'd be awesome.. I hate not knowing what I'm supposed to do.

Thanks a lot