If you are starting, you should definitely play the singleplayer. It will give you the basic and medium knowledge of the game. Don't be afraid of trying the Undiscovered Worlds after the original game, it will add you some skill too.

 

After that, you should indeed jump to online gaming - take no doubt that it is a really step jump you are taking, but if you played the singleplayer, you have better chances of not quitting Populous after some days. Why this? Mainly because if you never played this game and start playing online, you will probably lose 90% of the times, even against low rated players - and those 10% will mostly be because you have a good ally. Playing online sucessfully requires at least the basic skill that comes from the singleplayer. Note that there are exceptions, you can just jump to online without playing the SP but will be tough. Played singleplayer or not, online players are usually harder than campaign (unless they are low rankeds newbies), so you will have a hard time - but that's no reason to quit or be mad: you will only improve by losing - and trust me, you will lose alot. Losing is more important than winning, because when you lose you aqquire more information than when you win, because you done wrong things, or the enemy was smarter, so your brain will recognise those things you are missing.

Obviously you will be playing to win, and the more you play, the better you will become! After some months you can reach high ratings, if you are a good learner, and if you are competitive, you will get ti the top! After some weeks on playing Populous online, the singleplayer would surely get way easier than in the first times - but don't worry, there are hard Populous mods: actually, some mods contain levels that are harder that the top rating games in the matchmaker, so there is always improving in Populous.

Don't forget that even the best players lose, and the worst ones win - what matters is if you learn the lesson. Each detail, each information you can aqquire will be used later, and your skill will improve. As many say, Populous is easy to learn - but hard to master.