A familiar is a spirit that you are bonded with, something to hold near and dear to you, and a guide. A common familiar is a cat, or various animals and pets. Plants have spirits and can be familiars, too.
I suggest doing research into what a familiar is to you before finding one.
Once you have found a plant that you've felt a pull towards (For instance when I was researching familiars, thoughts of my cactus plant always came to mind), You need to show the plant that you care for it.
Re-pot your plant, used dirt from the original pot, and dirt from your home (ie, from your backyard or favorite place). Optionally, add the contents of used teabags, collect them for about a month before hand and mix the contents into the dirt mixture. Also optionally, you can add a stone in to the new pot, one that had an energy you want to give towards your plant familiar.
Carefully move your plant into its new home. You can talk to the plant spirit, be cautious and polite. Tell it your intent, the plant will respond when it feels ready.
Now, where to keep the plant? You could have it on your altar (or equivalent of), the windowsill of your room, wherever you feel most comfortable keeping your new familiar.
To communicate with the plant spirit, care for it, talk to your plant and keep it with you when you meditate.
Best of luck, and make sure that you know how to care for the plant species, do research! You want to have your new friend for a while.