Add half the extract at start of boil and the other half at 20 mons remaining in the boil... Make sure to balance the hops and the fermetables to get an IBU (Internat'l Bitterness Units) number that is appropriate to the style. You can use online beer calculators such as BrewBuilder or others. So suppose u are making a pale ale with 35 ibus, and you kick up the malt, u will notice the ibus have dropped and Original and Final Gravity have increased. Just add enough malt to get your Abv up to where u want it, and add enough hops to get your ibus correct. Remember early hop addition will add bitterness whereas late addition will ad aroma and flavor.
To sum up, whatever you add or subtract from the brew, make sure these things stay within the style profile: OG, FG, Abv, IBUs and also SRM which is color, but as a new brewer dont worry about that for now. If you dont know what these 5 things are, make sure to do some homework, because theyre all important. This website/ap is a great source of knowledge and opinion. Sorry for the long response...
And i agree, first couple of batches will be $hit until u get some experience, but trust me they get better! haha