it depends on a lot of things.
if you are making the transition from iron sights and want something useful at short
and longer range, perhaps try something lower power like a 3-9 or 2-7 power. the size of the objective depends on whether or not you want a compact, lighter scope or you want more usefulness in lower light.
i have a 6.5x20x40mm leupold VXIII and while its very nice for long range gophers its
useless if you call a coyote in close or a gopher pops up in a fencerow 20 yards away.
i used to always buy high-power scopes but lately ive actually been liking the lower power ones much better, and most of the time my scopes stay dialed down to the lowest power. especially if you are used to - and like - iron sights it may be in your best interest to get something with a low 2, 2.5 or 3 power setting.
as for reticle, if this is your first scope you can have a lot of fun with a mil-dot or ballistic plex reticle. mil-dot will let you estimate your range and thus help with holdover while being nearly as unobtrusive as a duplex reticle, ballistic plex by Burris has crosshairs below the main crosshair for 100, 200, 300, 400, 500 yards.
with the ballistic plex for most .223 loads youll be dead on at the 200 yard crosshair, +1 inch over with the 300 yard crosshair, -2 inches low on the 400 yard crosshair and it all goes downhill from there
(-11" at 500 yards and drops like a stone after that). the mil-dot is designed as a range-estimating tool, the ballistic-plex requires you to know your range but tells you how much to hold over.
some people will recommend that you buy the
most expensive scope you can afford right off. i disagree - id recommend since this is your first scope you get a mid-priced scope such as a Burris Fullfield II, Bushnell Elite 3200-4200, Leupold Rifleman/VX-II, etc. all of these scopes have full lifetime warrantys (Bushnell and Leupold have service centres in Canada, the Burris you have to send to the US). once you own one and use it for a few months you will be able to nail down what you like and dont like about it - and make a more experienced decision about your next scope.
for example it would suck if you put $950 of your money towards a Leupold VXIII 1.5-5x20mm IR scope and then realised that you do indeed want to spend most of your time shooting groundhogs in fields and want a high power scope with a large objective. or vice-versa.
the mid-priced scopes have made the most progress in terms of value for your money in the past decade, so IMO pick up a $250-400ish scope to begin with from a reputable manufacturer/line, enjoy it for a few months and if you are not happy with the type of scope you chose you can relegate it to a second rifle or sell it here in the EE for a pretty small loss.
IMO stay away from anything <$150 and from brands such as BSA, leapers, weaver, tasco, simmons, etc. some people will disagree and say that they have had great experiences with one of these scopes but they are the exception rather than the rule.
mil-dot:
ballistic-plex:
ballistic plex labels for different calibers:
http://www.burrisoptics.com/pdf/BPlxlabl.pdf
to learn more about mil-dots, simply pop 'how to use a mil-dot' in a google search and youll come up with tonnes of how-tos from simple to very in-depth.
