So my GF is getting an Xbox and Destiny this week, I was going to play from the start with her, or should I tell her to use her light to get to the taken King stuff?
If she's only planning to have one character then just go ahead and use the light to get up to speed faster. But if she wants to play everything from the start for the first character and might consider making alts in the future then she could hold onto it for that. Personally I'd go ahead and use it just to expedite the leveling process since it takes a ton of XP to reach max level. It'll still take tons of XP to max the subclass anyway.
Keep in mind you can't really outlevel enemies in Destiny like other RPG-ish games, so using the level boost wouldn't totally spoil the early missions or anything.
Looking for coming back to destiny (played a little of the original game and bought TTK) Leveled to lv 25 and want to play a lot more. Any tips for beginner players?
Hmm these are always hard questions to answer. Depends on what activities you enjoy, PvP, PvE or both. Generally I just recommend sampling all the different modes to see what you like best. Play through the new story missions (on hard difficulty for extra XP), run some heroic strikes, pick up different quest lines. Check out the different PvP modes if you have any interest in that, although solo queuing for PvP can be rough. The sooner you can get to Lv40 the sooner you can start grinding up your Light Level to get into end-game activities like raids, Trials, Challenge of Elders, etc. Also it'll make it easier to group up with people around here to run these activities.
Honestly I don't think there's too much important stuff for beginners to worry about, just get to max level and if you're still enjoying the game then you can worry more about the details. Probably not a very helpful answer, haha, but hopefully gives you a general idea.