This guide is here to help you level.
Choosing the right game mode is crucial. CTF modes (Capture The Flag) is probably the most preferred way as they allow you to get potentially unlimited kills without ending the match.
One CTF farming strategy is to make a custom match one difficulty level higher than you actually are (so if you're level 1 - play on easy mode, level 6 play on medium, ETC). Make the score to win 3 flags, and set up to have 3 allies and 6 enimies. This works really well for low to mid levels, so you can learn macro while still leveling up. You can always change the suggested options for maximum effectiveness.
This is extremely important. It's generally better to get guns that do damage from far away (Sniper rifles over melee weapons) and fire fast (assault rifles over magnums). But if you need to play strategicly, melee weapons or magnums can work well when you get used to them, just note that with a close range weapon it is recommended to have a long range secondary weapon like a pistol. Ergo, with a low ROF weapon it is best to use a fast-firing secondary like a machine pistol or a SMG.
Generally try to go for killstreaks that fit good in your strategy or work good in a certain level/map. For example, with a melee weapon it is smart to use smoke bomb, to teleport away if you face to many enemies at once. smoke bomb also works good on dominitation or capture the flag, because you can easily teleport back if you have the flag, or if you're lucky you can even teleport right to the flag!
Skills also depend highly on your strategy. A Medic is good with self revive to allow him to stay alive longer, or Charismatic to go streaking, or regenerative tissue to speed up regen. An Assassin can use shadow blend for "traps", laser sight for increased aiming, and blur to stay alive longer. Commandos with machine guns are advised to use ammo feed, although on small maps martyrdom is better and with explosives you might want to use flame resistance. A tank can use adrenaline to speed up regen, full protection to increase defence with shields, or iron will to resist slow big damage attacks.
Closing words Edit
These are just tips for leveling. Even with these tricks it will still take a lot of diligence and perseverance. Good luck and have fun but also stay carefull.