You can use either CarbonCopyCloner or SuperDuper to do what you want.
I recommend CCC.
It would help if you told us WHICH VERSION of the OS you're using.
You can do this:
1. Erase external SSD to Mac OS extended with journaling enabled (GUID partition format)
2. Connect to Mac and open CCC
3. You want your internal drive to be the source and the SSD to be the target
4. Now, you need to use CCC's feature that lets you "un-select" items YOU DON'T want to be copied over.
5. I would un-select the large libraries within your home folder (movies, music, pictures, etc.). Also any large folders that you may have created to store stuff in.
6. You DO want to copy over your home folders (without the items above).
7. Then... let CCC do its thing. It will clone over the necessary OS, apps, and basic home folder -- but leave "the large things" behind.
8. If CCC asks to clone over the recovery partition, YES, let it do this as well.
The idea here is to create an SSD boot drive that is "lean and clean", with plenty of free space "at the end".
This way it will boot quickly and run at its best.