Although it might sound egoistic at first glance, this one is a very important rule:

Stay away from the newbies!

Why is this so important? Very simple: because they act clumsy and they put you into danger with it, too. To name a few examples probably everyone knows from public playing:

• you get blocked by a team mate so you can’t strafe or escape and thus get shot or naded
• you get team damage while beeing in a firefight from a clumsy teammate behind you so you can’t aim anymore, you loose health and both – you and your teammate – are dead; you won’t grudge him his frag will you…!?
• you sneak through the way in order to hear if an opponent is aproaching; then a teammate comes loudly nearby, reveals your position and you both eat a panzerfaust-shot
• you walk behind a newbie who draws a nade in a dangerous area/situation, gets shot and does team damage by loosing the nade
• you get team damage because the noob throughs his nade behind you no matter if he does team damage by it
• you get team damage because you stand near a mine and someone else tramples through it
• some newbie engineer places a mine behind you and you trample through it while beeing in a firefight
• you got to walk behind a newbie in a narrow way, the newbie blocks the way to your next frag and gets shot without doing any harm to the opponent, you couldn’t shoot because you would have done teamdamage to your mate and you get shot, too “through” your mate
• you get splash-damage from a friendly newbie-pf
• you get team damage from a friendly mg
• ………

So all in all you better go frag with a good player or fragging on your own alone and you better only revive a newbie or an unknown mate if you

1. are absolutely sure the scene is clear or
2. you can fall back immediately after reviving into a safe place (e. g. by jumping out of a corner with the syringe in your hand, revive and immediately jump back into cover) or
3. if you can use his revive-shield to protect yourself effectively

Note that this isn’t egoistic at all but good for the team because the revived newbie would be dead again in a second and so would you and nothing has been won.