The basic functions of を, に／へ, and で

There are a few basic sentence structures that you learn with these particles.

location に／へ motion verb.
Ex: としょかんにいきます。／としょかんへいきます。Go to the library.
The に／へ here marks the destination you have.
NOTE: The particle へ is pronounced as え.

location で action verb.
Ex: うちでねます。Sleep at home.
The で here marks the location that you are doing something.

indirect object を action verb.
Ex: みずをのみます。Drink water.
The を here marks the indirect object, showing that you are doing something to something else.

You can combine the above two to make sentences like this:

location で indirect object を action verb.
Ex: レストランでばんごはんをたべます。Eat dinner at a restaurant.
NOTE: When combining these, you MUST keep the particles with the works they are marking.
レストランで／ばんごはんを／たべます。
Technically you can mix these around, as long as the action verb stays at the end, but I would not recommend it as it gets confusing and can sound a bit odd.

A LOT OF PEOPLE MIX UP に AND で
The easiest way I have found to remember which is which is that a location with に has a motion verb at the end and a location that has で has an action verb at the end.

This may change once you learn more complicated grammar, BUT if you are taking Japanese 120 and using Nakama 1a, REMEMBER THIS