Beginner friendly [Hek] build. All of the mods on this build are accessible without spending any platinum, and makes for a pretty potent early game build.

Point Blank, Shotgun Barrage, Hell's Chamber, and Chilling Grasp/Contagious Spread are obtainable through normal gameplay, and you will eventually come across them. These are the core mods that will amplify your damage greatly.

Vigilante Armaments are obtainable through Cetus Bounties (Level 20-40).

Toxic Barrage has a 25% chance to drop from killing Corrupted Vor.

Scattering Inferno is obtainable through Lua Spy missions. You may also get other crucial mods from Lua, such as Hell's Chamber.
Alternatively, you can get Blaze from Nightmare missions.

Frigid Blast is obtainable through Kuva Fortress Spy missions.
Alternatively, Chilling Reload, also from Nightmare missions.

You may replace Vigilante Armaments with Scattered Justice if you choose to advance your Steel Meridian standing.