I had much the same questions. I made some notes and guesses as I was trying things out. FWIW, I really like decking in SR Unlimited.

There are six skills associated with decking. Each skill has a rank from zero to eight. If you start the game with some points in decking, you have the option to get a beginner deck and some of these skills at level 1. You improve your skills by purchasing upgrades from the vendor in the Hub. Rank 1 in each skill costs 400Y, Rank 2 800Y, and (I assume) so on. There is no KP cost associated with the skills. The skill rank you can purchase is limited by your deck - the beginner deck (Sony CTY-360) can only handle skills up to level 2. You need to upgrade your deck to purchase higher ranks. The Kraftwerk deck lets you take levels 3 & 4, I'd guess that the Fuchi deck is needed for levels 5 & 6, and you'd need the Fairlight Excalibur to run levels 7 & 8.

The six skills are:

• Spoofing: Used to unlock access control nodes, granting further access into the matrix. This skill is one of the most commonly checked. I boost Spoofing and Exploit whenever I can.
  
• Exploit: Used to take control of a node. Notably, you can exploit a node that allows you to reset the alert status to zero. Everything you do in the matrix (moving, attacking, etc) increases the alert status, and when it reaches max level, you have to fight IC at every node. The ability to reset the alert level is quite handy. Again, I boost Spoofing and Exploit whenever I can.
  
• Masking: This is a passive skill. You sometimes see a message to the effect of, "You successfully masked your action, and the system takes no increased(?) notice of you."
  
• Browse: I think this helps you find the file you need for a mission. Maybe it helps with locating paydata - I don't know.
  
• Delete: Some missions require you to delete a particular file. Presumably this helps you do that better or more quickly.
  
• Edit: Some missions require you to edit a particular file. Presumably this helps you do that better or more quickly.

When you come to a node in cyberspace, it'll have a color (blue, green, etc.), a type (one of the skills listed above), and a rank. There's some kind of die roll / skill check, and if you succeed, you can use the node or proceed through the gate controlled by the node. I don't know what the colors mean or what effect they have on the skill roll. Getting your skill levels to the max allowed by your deck seems to help a lot on the die rolls.

Often when you attack an IC in the matrix, you will start a "trace". This is essentially a timer, and you need to kill whatever it is you are attacking before the time is up, or you will get booted from the matrix and need to restart the run. Your chance-to-hit in the matrix seems much lower than with comparable stats in DF or HK. If you are repeatedly getting booted because you can't kill something in time, increase your intelligence or decking. It'll give you a noticeable improvement in your to-hit chance, and can help you eliminate the IC before the trace expires. Programs that increase your to-hit chance and that increase the damage done are also helpful.

You can interact with some nodes in the matrix for digital loot, known as "paydata". It's worth spamming the node until your deck is full or you get a message that there's no paydata to be found. This loot is stored on your deck, and your deck can only hold a limited amount. Paydata can be sold to a data vendor - Serial Rogue at the Hub or Roscoe in Downtown. The price varies per file - from zero (occasionally for junk) to hundreds or thousands of nuyen. You can purchase upgrades to your deck's capacity from decking vendors (like Quick Jack in the Redmond Barrens). You don't lose the upgrades by changing to a different deck. Upgrading your capacity can significantly boost the amount of nuyen made on a matrix run.

Thanks. The skills only go up to 8? Some of what you said, I'd already figured out in the months since my original post, but the main thing I still need to know is how high are the skills supposed to have to be to be able to actually do any decking and have it be profitable. I've got them all up to at least 3, so almost halfway to max, and I still rarely succeed a check on a random node beyond the easy first check just to get into the node at all. Is grinding random nodes supposed to only be feasible once you've maxxed out everything and thus have nothing left to grind for?

A high intelligence/decking allows even ♥♥♥♥ tier decks and level 2 programs to go pretty far into yellow, its the orange nodes that require upgrades to at least a tier 3 deck and upgraded programs