"I want a sandwich."
Translation:Jeg vil ha et smørbrød.
When you see "want" in front of a substantive, it needs to be translated as "vil ha".
When you see "want to" in front of a verb, then it's translated to just "vil".
"Jeg vil ha et smørbrød."
"Jeg vil spise et smørbrød."
In both of these sentences vil functions as an auxiliary verb, which is why it needs the verb ha added when there is no other verb present for it to modify.
"En brødskive" usualy implies a slice of bread with topping, so an open-faced sandwich. If it's for your "matpakke", it could be sliced in two with topping in-between. It's simple, everyday food.
"Smørbrød" also tend to be open sandwiches, though it could refer to a regular sandwich consisting of two slices of bread as well. You can expect the topping(s) to be more elaborate.
"En sandwich" has actually made it into the dictionary, and refers to your typical American sandwich consisting of two or more slices of bread. However, it doesn't see much use.
Norwegians usually prefer open-faced sandwiches, so that's what you're most likely to be served here.