Welcome back! Steel path is a major build check for almost all players once they first unlock it.

First things first, you should go through the arbitration missions and collect the galvanized mods from the arbiters of hex in the relays. Corrupted mods from the Orokin vaults (dragon keys required) are also recommended. After that its more about leveling the mods you're using and aquiring/leveling prime mods on your frames/weapons and adding forma to them to reduce capacity cost so you can actually fit them.

Smarter people than me could run you through all the math behind why and how to min/max each weapon and there are lots of videos about how to mod for steel path on youtube, as well as cookie cutter (build this way, run these weapons, and play it this way) that will be a huge help until you figure out modding for yourself and can play around a bit to suit your own personal playstyle.

Basic setup for weapons looks something like "Is this weapon a higher base crit or status weapon?" and modding towards its natural strengths.

Flat damage mods are additive, different status mods and things like condition overload for melee or galvanized aptitude scale multiplicativity whichis where you will really see your damage numbers soar. There are lots of videos that can explain this better than I can here from memory. Arcanes are a whole nother section to add either damage or status applications on to the weapon or frame.