You have, in fact, posted in VNMaker, not in the MV section - easily done with the similarity in name.
[move]RPGMaker MV Support[/move]
Your query isn't entirely clear to me, but from what I think you mean, then you need to have an event on the map that the player interacts with in some way. In that event, you use the command 'Battle Processing' (3rd tab of the Event Command Menu) and specify which enemy troop you want the player to fight.
I suggest you have a look at Andar's compilation of
info for new users. One of the things he links to is a playable tutorial for Event Commands. Working through that will give you a very solid foundation in how to use events, and will save you a huge amount of time and frustration in the long run.