So the Sims card that is in the phone is gonna come out and I wont need that one anymore? THen just install the new one in that same slot, right?
No, you need the SIM card for the phone to communicate with the Metro 4G network as it contains info like your phone number and other vital communication info.
Leave that one alone as it is much different than a Micro SD card.
The SIM card holds info such as your phone number and what type of communication is supported, while the Micro SD card stores Music, Video, Apps, Contact info, etc... so if you buy another phone, you can put the Micro SD card in your new phone and have all of your apps, contacts, music, vids, etc...
Just be sure to save or move your music, vids, apps, contacts to your Micro SD card.
It's a good habit to save those things to the Micro SD Card and not the internal storage on the phone.
Keep in mind if you save your vital info, apps, pics, video, etc... to your Micro SD card and format the card at a later date, you will lose that info, so be careful not to format/erase your Micro SD card after you have written info you want to keep to it.
If at some point you buy a new phone, they will also give you a new SIM Card that they will program.
Leave the SIM Card alone.
The SIM Card will likely have "Metro PCS" and some numbers imprinted on it, while a Micro SD card will have a name like SanDisk or Kingston or Samsung on it, along with the capacity, such as 32GB
The SIM Card is located under the battery, near the top of the battery while the Micro SD card is also located under the battery it is located on the left side and slides out from a slot on the left side of the phone once the battery has been removed.
SIM Cards look something like this:
Micro SD Cards look something like this: