Dilly - The approach suggested by Stu (S&R) covers a very broad spectrum of songs. The important part of the system is essentially "if it doesn't sound right, try something different". How to judge that is a skill which can be learned as long as you have a reasonable ear for a tune.

In the meantime, remember that the quickest way to learn ANY skill is to study (watch, listen, ask, copy) other people who already have the skill. Most of this skill comes from recognising patterns you have heard or used previously. Other musicians have spent time learning patterns of notes and the chords which fit (or don't fit) - You can save a lot of time and mistakes by finding out what chords they would use.

Do not try to reinvent the wheel.

If someone plays a song, ask them nicely what chords they used, most of us are happy to jot something down on a piece of paper.

For a specific song, ASK here on Mudcat, you will usually get a dozen variants of different level of ability within a couple of hours.
COPY what others have learned from the combined hard work of composers and instrument players before them. Remember that any "mistakes" you can make have already been tried and discarded by thousands of others, who have passed on their experiences of what to avoid.

Start with something simple or straightforward. You have a better chance of understanding what is being done. If something works for ONE song, there will be a stack of other songs where the same pattern applies. Worry about complex changes once you have a grasp of the basic principles.

If you have a recording of the song with a chord accompaniment, listen over and over, then try to match chords so they sound like those on the recording. Sometimes professionals may use more obscure chords, or change between different chords, but often they don't. As a basic starting point, try to match ONE of the chords used. Once you have decided what it is, play along with the bits where that one chord appears. When you are happy with that, find and identify another chord in the sequence (forget about the first chord - it will just complicate things). Don't try to get them all at once, just leave gaps where the other chords are played.

Remember that the important thing about performing a song, is THE SONG (something which many technically clever players forget), the accompaniment should enhance not swamp the meaning of the words. Sometimes leaving leaving a "space" in your accompaniment can be more effective than trying to fill every second of the song with a chord.
Just because someone else uses a particular "clever" chord sequence doesn't mean that it cannot be played with something more basic. You can always add more as your confidence in your own ability increases.

MOST IMPORTANTLY - HAVE FUN!

Quack!
Geoff the Duck.