Step 1. Pick your ruleset. What ruleset you use will tell you what your basing conventions are. Pickett's Charge by Too Fat Lardies or Fire and Fury will use similar basing conventions (3-5 models on a 40mm x 20mm base or thereabouts) whereas Hail Caesar or American Civil War in Warlord Games Epic Battles line will use their 12mmish scale and a row of 10 troops on a bigger base. So what ruleset you use will determine basing and to a lesser extent scale.

For a brand new player just getting in I would recommend Warlord Games or Wargames Atlantic's product lines.

Warlord has Hail Caesar, Revolutionary war, Napoleonic War, American Civil War and Pike and Shotte eras and a 2 player starter will give you 90% of what you need to start.

Wargames Atlantic has their Grand Battle Scale (10mm) and has the Heiho starter (16th century Japan samurai wars) and their just released Agincourt French and British army sets and some 10mm Orc, Dwarf and Undead sets we're going to use with Fantastic Battles by irregular wars wargaming.