Hi, I remember having the same problems about 18 months ago, I guess I'm just somewhat ahead on the ol' learning curve.
For starters, you could play both songs with a DDUUD pattern. (D is for a downstrum, U for an upstrum).
Some very basic rhythm theory for you:
You're playing eighth notes, when you strum 8 times ( 4 up, 4 down: DUDUDUDU) over one measure. That's a little boring to play over songs, however, so in order to give it a slightly more dynamic feel, you leave out some of those eight strums: DUDUDUDU -> D_DU_UD_. The strums that are now underscores are being left out. Keep your strumming hand moving in an eigth note pattern, but don't touch the strings when you want to leave out a strum.
It works quite well for Knocking on heavens door. Play this once on the G, once on D and then twice on C or Am, then repeat.
ONce you have this down, which will take you no time at all, try to vary the pattern a little, but keep your strumming hand moving steadily. Simply miss different strums every time you go around the chords.
DDUUD works on pretty much all songs that are written in 4 time, but not on waltzes, 6/8, etc.
You could do a youtube search for "accoustic guitar lesson" to find easy lessons, helped me a lot, and it's easier to show stuff with a guitar than explaining in words.
Apart from that, keep playing!!!
English is not my native language, so please forgive me for not using accurate musical terms or sounding a little strange.
Cheers,
nc