You add the arguments using the client.
Click the "Options Tab" at the top and this will open the window where you can control several things about the client.
The bottom line will tell you the version of Java you're using and has a long push button labelled "Advanced Options". Click it.
You can change more things here but you want the 3rd line down, "Additional Java Parameters".
A couple of things to beware of...
1). If you run Java 1.8 you shouldn't need additional arguments, unless you are using a huge texture pack.
2). If you run Java 1.7 you will most likely need arguments. To match Java 1.8 (recommended for packs like Infinity and Direwolf20) you will want:
Minimum Ram = 2GB
Maximum Ram = 4GB
Perm. Gen. = 256MB
[Edit: MAKE SURE YOU HAVE ENOUGH RAM TO GO AROUND!!! It won't do any good to allocate 4GB of Ram if you only have 2GB installed. Windows (assumption) will need 1.5-2GB ram before any you allocate to Minecraft.
I find no problems having 8GB in my system but have heard of people running with only 4GB by cutting down on game settings like render distance, clouds, etc. in the MC Video settings]
Good luck!