Farsite Ship Fitting depends on several factors. Your ship have limited number of slots to equip Modules.
Here is a list of First Edition Farsite Ships that were available adter store open.
On Ship Menu you can see Ship Specifications which shows what Modules you can fit. Starter ship has only 1 slot for Type-I item. Not all items are weapons. Mining laser allows you to gather asteroid fields in space. When you have that module equipped, you can not equip Rail weapon, because your ship does not have enough Type-I slots. Every item or Module have 2 requirements, size of item (on image S), and Slot level Type (on image TYPE-I). You can fit Module only on slot which support that Module.
Ship have limited quantity of Power it can produce. Starter ship Morgan Moon A-1 have 144 maximim Power. Every item have different Power Consumption. If there is not enough power, that means Modules wont work, they are then just dead weight.
Farsite Ship Fitting changes gameplay dramatically. Same ship can be fitted to serve as fighter ship, miner, hauler. You can have multiple ships of any type you want in your hangar. As much as you can afford. Every ship with all its modules have value in cryptocurrency. You can build ships just to sell them, and save other people time, woho would like to have such service.