MODFLOW is a software for groundwater modeling developed by the USGS. The software is capable of representing conditions related to groundwater flow such as evapotranspiration, recharge, drainage, river interaction among others. Its finite different approach gives the capability to calculate the flow regime with exceptional control on water budget discrepancy. With MODFLOW one can rely on that the flow going in the model is the same as the flow going out of it, from the local to regional scale.
It has been a continuous development on different modules and tools for groundwater modeling with MODFLOW. New developments on MODFLOW includes the MODFLOW 6 version capable of unstructured grid refinement, Flopy a Python library to build and run MODFLOW models and MT3D an improved transport solver.
Your relation with documentation
One smart choice to learn MODFLOW (and any other software) is to read the documentation. It might be that the level of review would be new to you. You need a couple of hours to know the new tools and gadgets of a smartphone, this is not the case when you learn numerical modeling. Groundwater modeling is by no means a process of setting up some geometry and boundary conditions and making reports of the model simulations, it is much more than that.
Most probably on your first modeling attempt you wont finish a complete run since you found a numerical unconvergence, wrong boundary configuration, isolated cells, crossing layers, solver relaxation in between thousands of alternatives. There is no way to have a recipe to solve each error, so the best way to save you from total frustration is to learn how the model really works and you can achieve this by reading the documentation.
Here are some useful links relates to groundwater modeling with MODFLOW
Links to MODFLOW core documentation:
MODFLOW and Related Programs
Online guide to MODFLOW 2005:
New and popular MODFLOW related programs
MODFLOW 6 - New MODFLOW version
MT3D-USGS - Mass transport modelling
MODPATH 7 - Particle tracking modelling
MODFLOW NWT - Newton formulation for MODFLOW 2005
Links to MODEL MUSE and other MODFLOW input/output data processing programs
A Graphical User Interface for MODFLOW-2005, MODFLOW-LGR, MODFLOW-NWT, MT3DMS, PHAST, MODPATH, and ZONEBUDGET
A Program For Three-Dimensional Visualization of Ground-water Model Results
A program to create specialized graphs used in groundwater studies
A Python package for creating, and running MODFLOW-based models
Post by Hatarilabs
More than 40 tutorials of MODFLOW can be found in this special selection of posts by Hatarilabs:
Articles of hydrogeology and topics related to groundwater modeling can be found here:
Blogs and forums
Groundwater Modeling Forum on Linkedin
MODFLOW Users Group on Google Groups
Groundwater Modeling Blog on Blogspot