Thanks all. I decided to buy the codex and saga of the beast. I’ve also downloaded BattleScribe and the core rules, but there’s obviously a lot else in the rule book that I need to understand, eg about the different types of detachments, battle forged armies, etc. I have to say that with the various Primaris units I don’t see much use for normal ,Raines anymore, especially Grey Hunters, which is a shame. Also my old squads aren’t even close to game legal now (too many power weapons and power fists, which was all legal in 2nd ed) so I’ll be reading some tactica and trying to work out what I want/need for a current army.
Shi An De, you may want to check out this Buying Guide that I've put together for new (or returning) players. I've recently updated it, based on what we've learned in the past week or so about upcoming new units. It'll give you a good place to start for putting together a new Space Wolves army.
You don't really need to worry much about the different types of Detachments, outside of the fact that 9 times out of 10, we're all mostly going to be fielding a single Battalion Detachment, which is very much like the old Force Organization Charts that we used to be allocated to build armies with in previous editions.
In a Battalion, you get a minimum 2, but up to 3 Headquarters choices, so you just pick units with the HQ symbol (the Skull) in the upper left-hand corner of the data sheet. You have to fill a minimum of 3 Troops, but up to no more than 6 Troops choices, so you just look for units with the canted Triangle in the upper left-hand corner; if you're building from scratch (perhaps using that linked buying guide above), then you'll just want a couple minimum sized Intercessor and Assault Intercessor packs to fill your Troops requirements.
You can have up to 6 Elite choices (with no minimum), so here is where you're going to select units with the Elite symbol (the Cross with a Skull in the middle), like the new Judiciar model, Murderfang, Wulfen, a Redemptor Dreadnought, and/or Invictor Tactical Warsuits.
You can have up to 3 Heavy Support choices (the Blast looking icon), so here's where you'll put either Eradicator or Eliminator packs, or a Repulsor Executioner.
You can have up to 3 Fast Attack choices (the Lightning Bolt) and here you'll just want a unit of Outriders, for now.
Being Battleforged basically just means that you used a Detachment to create your army, and every unit is from the same Faction (Space Wolves). When you do this, then you get the benefit of certain faction special rules, as well as a free extra Command Point that you get in your Command Phase.
Don't hesitate to ask if you have more questions.