If you're completely new to Prophecies, best idea would be to play through, yeah.
You might have trouble in parts though, as neither Prophecies or Factions give you access to heroes. While there's very little or nothing in the PvE storylines of these two that is impossible to do with only henchies, many people have trouble henching their way through some quests and missions.
At least a fair amount of people still play in all the campaigns, but many of us are sporting heroes and don't join up with PuGs as much as we did before heroes were available. If you ask nicely (i. e. speak to people, ask questions, ask for help, come off as a human being rather than a bot programmed to spam LFG!!!!!, etc.) I think you can still find help if you get stuck on something.
You'll probably have to look harder for help on secondary quests, as fewer people do every quest on all their characters. But in the end I still say it's worth it to see everything at least once.
Factions and the games beyond are set up very well to allow you to take characters from different campaigns and play through most of the storyline there. What you miss out on in Factions or Nightfall with a Tyrian character is quest progression on the starter islands, Shing Jea and Istan. A Tyrian character can travel there and play the missions, but they don't get any primary or secondary quests in those areas. Similarly, non-Tyrian characters can travel to Tyria and explore everything but Pre-Searing. They don't get any primary or secondary quests before Lion's Arch in the story progression however. There's a lot more storyline in Prophecies between pre-searing and LA than there was on the starter islands of other campaigns. Seems that way to me anyway.
If you don't like the idea of making a new character to play entirely through each campaign, I'd suggest making a Tyrian character and playing through Tyria, then Factions, then Nightfall (if you get it,) then GWEN. You're not tied to that order, but I always figure why not go in order if you can? You can sample the starting areas of the other campaigns by starting a new character there and at least playing up to the point in the story where the Tyrian character arrived. Then, you've covered almost everything story-wise.

And at worst you have a Factions and a Nightfall PvE mule character to hold the ranger's toys.
Even if you're new, I'd recommend taking your Tyrian ranger to Factions once you get to LA and grabbing max armor. True, it's not following the Tyrian storyline exactly. But it saves you about 4 steps in buying higher armor levels before you can get max armor in Prophecies.
And if you never got into playing Factions but are addicted to Prophecies, then I'd guess you'll like Nightfall and GWEN. IMO Prophecies is still the best, but Nightfall and GWEN PvE feel a lot closer to the ah-this-is-so-cool!!!ness of Prophecies than Factions did.
Oh, and don't think in terms of "First level up my ranger to 20 and then play through." Just play, leveling happens as you go. The only time I'd ever stop and just work on leveling up in Prophecies would be if I were a lower level than the local henchies. If you're exploring and doing secondary quests, you should be fine.
Cheers, Congrats & Enjoy!
Luny