Therer are a lot of different useful packages for julia. All official
Packages can be found at https://juliaobserver.com/packages (I think)

Here are a couple of good packages i use a lot.

IJulia:
This package is required for using Julia in jupyter.

LinearAlgebra:
This package comes with julia without any need for installation and includes
a lot of linear algebra functionality, comparable to numpy.linalg.

Statistics:
Distributions, mean, standard deviations, etc...

Plots:
Creates nice plots. Has support for multiple backends (including pyplot).

PyCall:
makes it possible to import python packages into julia and write python code
in julia files. Anytime there exists a useful python package and no julia
equivalent, this package saves lives.

Primes:
I do not know what magic they put into making this package, but I have never
seen a so fast funtion for checking if a number is prime ever.
The package is useful for working with prime numbers.

Memoize:
A useful package for automatically memoizing a function. For people taking
algorithms and datastructures, this will make more sence later in the course.

Images:
Useful to handle images. Terrible slow to load the package, but once loaded
it is quite fast and very featureful.

SymPy:
A wrapper for pythons sympy package which is arguably much better than
sympy itself.

Printf:
Wrapper for the C- printf style functions for fast string formating.

CSV:
Package for reading/writing .csv (comma seperated values) files.

JSON:
Package for reading/writing .json files.