First, welcome to Shadowrun. Second, if you haven't, I highly recomend geting the Data Trails source book if you plan to be a decker. It has lots of goodies to make your job easier.
Now on to the meat. While the previous comments are right, you should try to avoid cybercombat, you do not want to neglect it. I can't tell you how many times a run has gone sideways and I needed to bring down the hammer on a spider or defense node/ICE. For that eventuality I recomend having the configurator program with a full offensive load out preprogramed. That way you can be ready to rumble with a single action instead of having to spend several combat rounds switching everything out.
Next, lets talk decks. While the ones in the core book all have variable arrays, Data Trails has "prebuilt" decks with fixed attributes. These are generally cheaper than an array deck of simmilar device rating. The downside is of course that if you bring the wrond type of deck to the party you are going to be screwed.
For main skills I generally put the priority as hacking, computer, cybercombat/hardware, electronic warfare, and finally software. A note, having a decent hardware skill is a necessity for changing the ownership of... "acquired" devices. A minimum pool of 15 is recommended to allow you to buy hits to accomplish this. Trust me. You do not want your looted gear squacking it location and leading its previous owners back to your doorstep.
Secondary skills. This one is a little trickier. Perception, as has been mentioned, is highly valuable(though matrix perception is based on computer); and you most definitely need a weapon skill. My recommendation however is for pistols. They are cheaper, easier to acquire, and if you are busted carrying one its easier to get the cops to look the other way. Gymnastics is a solid choice as it grants you better defense in combat. For your secondary specialty with logic being your main attribute you should look towards other logic bassed skills. First aid is useful, but can be covered with a high rating med kit. The other medical skills are very rarely ever useful. Demolitions... well, I might be biased, but I've had to default defuse far too many bombs in this game. For the rest weigh them carefully against what you want to do, and what the rest of your team is lacking.
In combat deckers can actually be quite useful. If you get marks on an enemy's pan you can do all kinds of things to screw with them. The most common are ejecting the clips in their guns, bricking cybereyes/optics, and the ever popular slagging of drones. Of course sometimes a more subltle touch can get you better results like, oh, say, tweaking their targeting systems so that they shoot their allies instead of you.
On to gear. Don't forget to get basic matrix gear; trodes, ar gloves, and the like. I can't stress that enough. For hacking equipment it pays to look at dongles from Data Trails. They can be used to give comlinks matrix attributes that they would otherwise lack; for quick and dirty hacking when you don't have your deck. The same book also has a number of deck/comlink modifications. When it comes to 'ware cerebral boosters are a good basic choice. If you plan to do any significant AR decking synaptic boosters/wired reflexes are almost required.
Most of the best qualities have been covered, but I'd like to add Go Big or Go Home. It is ideal for those situations when you need a lot of marks on a target in a hurry.
Hm... that's probably enough for basic decker advice. Once you've made a character and gone on a few runs you can start getting into the complicated stuff.
