One caution about snipers - they are a niche role. It's better to be good at ranged combat in general - in fact, you can get away with being slightly worse at sniping, because you don't need as many dice to hit an unaware target.
Dice pools in combat have three components: Attribute, skill, and modifiers.
For Attribute, you use Agility for shooting, as well as numerous other skills. You want an Agility of 5, because getting it at the base maximum of 6 is not worth the 25 points it costs. You will hear the term "soft-maxing" a lot here. What it means is getting your most important Attribute(s) at one point under the maximum, since there is that balloon cost for getting it to the maximum. After you have bought your "natural" rating, you can increase it by technological augmentations or by magic. Shadowrun is a game with a strong transhumanist theme, which is reflected in the fact that augmentation and magic can both give you comparatively cheap boosts that take you over the level of ordinary humanity. My recommendation for a shooter is to get muscle toner at rating: 2 (if you have Runner's Companion, there is a quality called Restricted Gear that lets you go over the normal starting limits for a piece of gear - taking that quality and then getting the same muscle toner at rating: 4 is a common tactic for players). If you are playing an adept, you can either get the same thing (it will cost you a point of magic loss, but is still usually worth it), or, my favorite combo for "pure" adepts - get improved Attribute/Agility rating: 1 to raise your base Agility to 6, then get Attribute boost/Agility, also at rating: 1 (don't get it higher, because you roll magic + rating to temporarily increase your Agility, then resist Drain equal to the rating - every point past rating: 1 gives you an additional point of Drain damage to resist in exchange for only 1 extra dice; not worth it).
For skills, you can either get the firearms skill group at 4, or get your primary shooting skill at 6 with a specialization. The former is more cost-effective, eventually, if you plan on using all three skills (automatics, longarms, and pistols), but compared to the skill and specialization, you will be starting out with 4 dice less. To improve a skill, you can get reflex recorders, either for an individual skill or a skill group. Adepts can get the improved ability power, which can improve an individual skill by up to 1.5 x its base rating - up to rating: 3 for a rating: 6 skill.
For dice pool modifiers, a relatively cheap modification is a smartlink. You need to modify your gun (some guns come with a smartlink as standard), and modify either cybernetic eyes, or a visual device such as glasses or goggles. It gives +2 dice. I would also recommend getting a skinlink modifier to your smartlink, which means you can't remote-fire your gun, but which makes it unhackable from a distance.
