Play against robots first, then increase difficulty and eventually play online
i would say go straight to online, pick a class and get diamond skins. Also do challenges and get the titles. You get so many points doing the challenges and titles.
Seriously? Who cares about weapon skin(s)? Unlocking Gold/Diamond skins doesn't make you a good player.
Earning camos let's you to learn each weapons pros an cons, for me lots of ppl rate the MSMC but to me it's a pile o' junk the SMR is difficult to get used to but it's better then a FAL when you get used to it. All depends on your style an map which weapon is best do the challenges and learn for yourself.
Getting gold skins in and of itself will not make you a great player. However, trying every weapon and all the attachments for it and how they change a weapon's characteristics WILL allow you to choose the right load out for the job at hand. For instance, when is the last time you ran with a SMR & ACOG on Hijacked? Probably never. How would a new player know that unless he had tried an SMR with ACOG and new how slow it is to ADS to fire a single round on a map that requires hip firing and how much your filed of view is blocked by an ACOG? How about which LMG has the recoil eliminated when you use a TF? You need to try all the combinations and give them more than one match to learn the positives and negatives of each weapon.
Along the way, every time you complete another step toward gold, you also get character XP to rank you up. Your final kill with a weapon gets you 20,000 XP (5 +5 + 10) If you let it sit one kill away from gold until you prestige, you skip all the low levels where you cannot even create custom classes, especially if you match it up to a double XP weekend.