One thing no one mentioned and i'm not sure if is mentioned in the newbie FAQ ( i was a noob before that existed and got a handle on the game before it was made and thus never read it) but with weapon slots, in addition to small medium large, and universal, missile, ballistic, and energy slots some ships have built in slots that CANNOT be changed ever, the thermal pulse cannons on the Onslaught capital ship are the only example i can think of atm, in some mods you'll find ships have Hidden slots, these are identical to normal weapon slots but in combat are completely invisible and hide the weapon as well, i'm unaware of any vanilla ships with hidden slots save most fighters.
When mounting weapons onto your slots in the loadouts the size of the slot obviously determines which weapons can go into it, however slots aren't perfectly exclusive, for example in a medium ballistic slot instead of having a medium ballistic you can instead mount a small ballistic if you think your ship needs one of those more than a medium weapon, however this only works one weapon size down( no small weapons in large slots) and does NOT go up ie no mounting large weapons on medium slots.
One last thing, make sure when your refitting ships that you try out your design in the simulation before committing ESPECIALLY if your pressed for cash and are refitting at a station, because if you decide a design doesn't work out while still in the refit screen you get refunded the FULL price of the weapons, whereas if you commit to it and leave the refit screen you can only get back a percent of your money.
okay i lied THIS is the last thing, Welcome to the forums, have fun!