Best way to learn before going online?

Do try to beat it on hard as that would allow you to improve your set up patterns. After that just jump right in.

Well, you can always read around the strategy forums for tips and suggestions.  If you can't beat hard yet, you'll probably get demolished online.  If you're willing to take a few lumps, then go ahead and give it a try.  If not, you'll need to work on your game a bit.

Here are some specific skills you can work on to improve your game:

1) Manual Scouting:  The auto-scout AI is usually insufficient and won't reveal the solar system quickly enough.  Manually scouting, at least until you've located all nearby enemies, is always a great use of your time.  Try to spread out your scouts so you cover ground evenly and minimize any backtracking.  The most important thing is to quickly identify key tactical planets so you can get there and colonize them before the enemy does.

2) Early colonization and expansion:  Learning to expand quickly and efficiently is extremely important, both for getting your early economy running (which will be primarily driven by tax and extractor) and for acquiring strategic planets before the enemy.  A lot of this comes down to using a minimum amount of units to capture planets.  One great tactic is to kill the Krosovs in the militia, colonize the planet, then build a turret to clean up the remaining militia (note: you'll also need to kill the Javelis LRM because they outrange turrets, but you can do this after colonizing).  Counter-intuitively, the scout is the best unit for fighting militia, since its attack-type is effective against the Javelis LRM and the Krosov siege frigate... the two types of units you must remove before leaving a turret to finish the job.

I'll add that you usually want to colonize in a bee-line towards the enemy.  You can always backfill neutral worlds at a later point in time, but it's going to be very difficult to dislodge an enemy once he gets settled in.

3) Rush fleets:  Players will quickly establish large fleets with the anticipation of fighting over key strategic worlds.  You need to know how far away the enemy is, estimate how long it will take before you meet them in battle, and have a fleet ready for that event.  Generally, you want to go lean on techs early-game and focus more on getting strong unit prototypes on the field.  For TEC, an ideal combo would be the Javelis LRM + Garda Flak Frigate + Hoshiko Robotics Cruiser.  The Sova is the best capital ship to support this.  Good fleets offer the best balance of durability and firepower, so you can't invest too heavily in capital ships; they are individually strong, but if you punch the numbers they have substantially less damage output and hull/shield than an equivalent expenditure in regular frigates.

Remember, you need to survive the rush-game to get to the late-game, so learn how to quickly amass a strong fighting fleet.

Of course, there's a whole range of tactics to employ, but short of turning this into a major article, I'll stop there.  If you have any specific questions, ask away.

You should also look at some of the replays floating around here. they should some of the best players at their best.  Watching just the first 20 min or so would very educational.

Here's a link to a replay of a recently played epic game on the Diplomacy expansion:

https://forums.sinsofasolarempire.com/405557

I believe an old saying has merit here, practice, practice, practice.

Auto-scouting is useful on a small map.

Always manual-scout until you've found your enemy.  This is critical for knowing if a rush is coming and how to pace your colonization and fleet-building.