This issue comes comes up way more often than it needs to, so I decided to write way too many words about how to tell other people where you and/or targets are. While this initially sounds like a simple enough concept, far too often people in fleets and corp chat will say things like "OMG!!!! I've been attacked! Help!!!111!!!6!". This sort of announcement provides less than zero information, and does nothing but makes you appear to be a panicky moron.
Where are you?
The first thing you need to know is how to tell others, who can't see what you're seeing, what the current grid looks like. To do that, you need to be able to tell them where things are. Merely saying there's a target 300km off station does nothing to tell anyone not on that grid where the target is - a 300km sphere around an object covers an awful lot of space. Which direction are they in?
In real space there are no absolute directions. In Eve, there are, because CCP built the game around a submarine simulation (not a joke). There is a real up and down in Eve, and you can always refer to objects as above or below another and make perfect sense to someone who understands the engine.
There are no easy reference points to give for the horizontal plane, and thus describing them must be done by convention. The most common way to describe where an object is either based on the direction of a station undock, or if off station grid, based on the direction of the sun. The undock of a station is usually considered to be the 'front'; similarly the side of a gate facing the sun is normally called its 'front'. As an example, the insta-undock bookmarks for Blue HQ are located in 'front' of it; the 'left' of BHQ lies between the sun and the Obanen gate; the 'right' is vaguely in the direction of the closest planet, and the 'back' is at a slight angle to the Poinen and Josameto gates. Most (sadly, not all) of the corp bookmarks are labeled in this fashion.
With this information, you can tell your fleet where you and/or some targets are. There is an enormous difference in saying there's a cruiser 200 off the gate, and saying its 200 above the gate. The latter lets people off-grid warp to any bookmarks they may have in that location without needing to see them.
Who are they?
As important as telling people where a target is, is telling them what that target is. If you go off by yourself and get attacked, simply yelling at your fleet to come save you can be a good way to get your fleet slaughtered. Begging your 10 man cruiser down fleet to save your dessie from a 20 man BC fleet without telling them that's what they're warping into will almost always result in the total destruction of your fleet, and everyone in that fleet hating you and thinking you're a moron.
Learn which ship hulls are which, and pass that information on. In the previous example, there's very likely no way for your fleet to save you; the only thing they can do is get themselves killed. On the other hand, if the only ships there are 3 BCs, your fleet can very likely kill them all if you stay alive long enough. Reporting that accurately will very likely get them there faster.
It can also be important to inform you fleet of what you're in. A destroyer against multiple BCs is dead in seconds, and not worth saving. A tanked BC against multiple BCs may be able to stay alive long enough for reinforcements to arrive.
Proper ship reporting also avoids overkill. A panicked request for help might get your entire fleet to come to your aid... but if they land to find that your Vexor is fighting a solo T1 frig, they're not going to think well of you.
What are they doing?
You need to be able to tell your fleet/corp all of this information as clearly as possible as quickly as possible. "Warp to me!" is not helpful. Learn to talk in the third person (yes, it will still sound odd after years of playing), and give the location of where they're warping so people know what to expect.
When scouting, don't just yell comments like "There's tons of them at the gate!" What gate? What ships are 'them' in? Where around the gate are 'they'? How many is 'tons'? A far more useful report would be, "Check, check. Blue(Red) fleet 200 down off Poinen gate in Jos, roughly a dozen mixed cruisers, 3 frigates on gate itself. A handful of others in local." This tells your FC there's anywhere from 15-20 ships involved in a potential fight and that the opposing FC is playing it safe and doesn't want his big ships on the gate but is keeping tackle frigs there. This gives your FC options, both in the sense that he has a better idea of whether it will be a fair fight, but also gives him a choice to try to drop your fleet on top of their fleet if he has the right bookmarks.
The FC may ask for specific ship hull types, to get an idea of what tactics the enemy will be using, but for the initial report this often takes too much time for you to say unless its very obvious. If more than half the enemy fleet is in Mallers, then reporting a Maller wall is essential. If there's a number of tier 3 snipers on grid, that must be mentioned as soon as possible. If the enemy fleet is a complete mix of different hulls, then just saying 'mixed dessies down' will convey the information well enough.
Keep an eye on Local Chat. When reporting hostiles, also mention how many hostiles are in local, and if that number has changed lately. There's often at least 2 or 3 people perma-afk in any of our systems: they can be safely ignored. On the other hand, if ten more of the opposing side enter system while you're describing the 5 ships on station, your FC is really going to want to know that.
Lastly, if you have any questions about what information is important, ask your FC. Every FC has slightly different styles, and the composition of your fleet will change what information they'll give priority to. If there's a lot of things going on, they may ask you to report in a very specific way to reduce their mental overhead. Please do so. Remember, if they're not on grid, they can't see what's going on. You need to be able to give them a clear enough picture, with words alone, that they can construct an image of the battlefield in their head, and then act on that.