1) Find a local unit to fall in with before you buy a bunch of gear and decide that the hobby isn't for you.
2) Research the unit's history, or the history of a unit that you would like to re-create.
3) Tailor your impression to what that research says, and not what the guys that have been doing that impression forever wear.
4) Shop used, or learn to sew.
You're going to find out quickly that impression quality varies considerably from unit to unit, so take your time to find a group you like. Authentic vendors are more expensive for some items than your mainstream quality gear, but it is worth the investment, especially if you end up going campaigner after you've already kitted yourself out in subpar gear.
Cheers and Welcome to the Hobby,
Garrett Glover