Starting out growing cannabis is arguably the most difficult stage relative to growing experience. Everyone remembers their first grow for a number of reasons, perhaps the excitement of starting to grow your own marijuana whether for recreational or medical use but also the fear, chiefly the fear of messing up and risking wasting your time and money.
A solid foundation is crucial to any grow, and the first step is buying your equipment. It is important that you take into account the space in which you are growing when buying equipment as this will dictate the size of the items you are buying and also the quantities. So, if you're buying a grow tent then ensure that it is able to fit into the space you have designated for your grow. Or if you're buying lights or fans, then ensure that you have all the correct sockets and outputs to supply your growing equipment with all the juice it needs. 
Also, thinking about the nutrients that your plants will need throughout the different stages of the growth cycle allows you to plan ahead. A lot of soils come with pre-mixed nutrients that provide your plants with basic nutrients needed for the early stages.
On top of all of this, be mindful of your budget, look for areas where you can save so that you have more income to invest in areas where you require something a bit more expensive. Overall, ensure that you are not breaking the bank and that your marijuana grow remains affordable so that you can sustain it.