plan your trip in advance (it's fun) but don't push it too far either (like buying all the passes in advances).
Leave some room to improvisation + these are not necessarily the best bargain compared with what you'll find on the spot (prices and flexibility).
Unless you've never been in a big city, you won't be lost.
Here is a tip that may come in helpful since you'll walk a lot (it's the best way to discover the city, which is not that big (you can also try the almost free rental bikes "velib" system that you'll find everywhere, just memorize your itinerary in advance and be extra-careful (and don't try to drive it like the parisians do - same as a pedestrian : it takes year to know how to do it and it's still dangerous...)) :
- once you're here, buy a real map of Paris (a booklet actually, one page per arrondissement), with all the street names. It's worth the (small : a few euros) investment).
- now, for the orientation : there is a system, that is related to the river (la Seine). Houses street numbers work like this :
(i) streets parallel to the river : numbers go up in the same direction as the flow of the river (eg : east towards west)
(ii) streets perpendicular to the river : numbers go up as you are going away from the river (number 1 is close to the river, number 100 is aaway from it)
(iii) even and odds are continuous, on each side of the street
(iv) right bank is the bank that's on your right if you are (supposedly) in the river looking in the direction of the flow of the river, left bank is on your left.
Once you know that, you can't get lost. Of course it's easier if you have a slight notion of where the river is, but you can also deduce it from the position of the numbers of streets (if you know on which bank you are, and you are at a cross roads !).
Anyway, you can also not bother, and ask people in the street for directions. provided you say at least "bonjour", and try not to ask the people who are walking the super-fast parisian walk (meaning : don't stop me, I'm going to walk straight in that green trafic light), this is perfectly usual.