Perhaps you are tempted to play riffs or 'tunes' before you know your scales and chords and before your fingers have the strength and dexterity needed. In any case, every rock band relies on its rhythm guitar. Keep soloing for later.
Learn basic major and minor chords. Practice for 30 minutes or so each day. Overdo it and you may burn out your enthusiasm. Be patient.
Learn scales - keep it simple at first and use pentatonic. Learn some 5th chords too for strumming to rock while your finger dexterity is still improving. Practice as above.
Use the scales as finger stretching exercises. In fact, you could just practice gently stretched down the frets between each pair of fingers to gradually loosen them. If a stretch hurts, leave it out for a while. Keep your fretting finger nails very short, by the way.
Learn about the CAGED system. It will help you get to use scales and chords all over the fretboard. Once you have that licked, learning a riff will be easier because you will understand how it is constructed and you will be able to improvise your own solos.
If you don't know any theory, get a book. The Complete Idiot's Guide To Music Theory has all you will need to know at probably much more. The guitar is a musical instrument and will give of it's best only if its musician understands what it can do and why.
The acoustic guitar can be hard on the finger tips until callouses build up and some barred chords such as F Major can be difficult to ring out well until finger strength builds up. You will probably find an electric guitar much easier. That and a practice amp. with a gain level control will also give you a better sound for the rock music that you seem to enjoy.
Get a good teacher. There's no substitute for 1 to 1 personal tuition tailored to suit your needs and progress.
Enjoy it! Don't make it a chore.
Some links for you:
http://www.cagedguitarsystem.net/
http://www.guitarnoise.com/ Google 'free online guitar lessons' for more.