Rythm

First look carefully at the song and make sure that you can hear the whole song in your head.

In general strumming pattern are used to sketch out the tune. Some songs are recorded with a simple strum pattern but most pop groups use a more complex rhythm. However that should not stop you playing along to the songs using a strumming pattern.
Many solo guitarist rely on strumming patterns in at least part  of their repetoire.

First you need to try to divide the song into "bars". These are the measures of time which govern the song structure. Bars are sections of the song which each last a equal amount of time. The precise length of time will vary from song to song.  A bar might last for 2 seconds for a rock song and 5 seconds for a slow ballad.

Think about whether this is a ballad or a uptempo rock song.

It may help by starting to play equally spaced up and down strokes ( four downstrokes in every measure). This will help you find the appropriate tempo and to work out where the chords change

Next you have to try to work out the rate of chord change in the song

You might start by assuming that the chords change at the start of each measure. However there may be parts of the song where it becomes apparent that the same chord last for several measures. You may also find that there are parts of the song where the rate of chord changes picks up a bit. Two chords per bar is common.

For most rock songs each measure of music will be divided into four beats. Not all beats may be played and the guitarist may subdivide the beats but these song nethetheless have an underlying 1 2 3 4 beat rythm. The first beat in the measure typically has a very clear emphasis.   This is called "common time" or 4/4 time.   

Some advice now on strumming patterns. We can''t stay with downstrokes for ever.

A word  first about the notation used below

V indicates a down stroke
n indicates an upstroke

The first pattern is a common time pattern known simply as down strokes.  I mentioned earlier. These are four equal downstokes in each measure

When you  raise your hand to begin the next down stroke make sure that it does not touch the strings of the guitar on its way up but keep the speed of your arm pretty constant 

1...&...2...&...3...&...4...&
V........V........V.......V....

The second pattern is called eights 

It includes an upstroke between each downstroke

1...&..2...&...3...&...4...&...
V...n...V...n..V...n...V...n...

The next patterns are popular variations on eights. This seems to be popular with Chordies

1..&...2...&...3...&...4...&
V.......V...n...V...n...V...n

Many songs alternate these patterns between measures. So for example a song might start with one eights pattern and replace that in the second bar with another before reverting to the first pattern

Now the more common variations.

There are many songs which use Sixteenths. This divides the each quarter beat into four

1...e...&...a...2...e...&...a...3...e...&...a....4...e...&...a
V...n...V...n...V...n...V...n...V...n...V...n...V...n...V...n

Many of these songs are quite funky. It is therefore rare that the guitarist simply strums all the notes in a sixteenth pattern. Usually many of the sixteenths strums will be missed leaving rests.   

You also come across songs in Waltz time or 3/4 time. This is very  different to common time. The underlying beat is 1,2,3. It is relatively rare in rock music but more common in country and folk. The most basic Waltz strum pattern is fairly straightforward

1...&.. 2...&..3...&
V........V......V....

Cheers for the advice guys, I got onto www.powertabs.net, and they had truckloads of music tabbed and with music notation which is helping but as you gays say it will come with practise