A fantastic question!
Just the recognition that this is something worth researching and learning will make you a fantastic brewer in the end.
I have always been a big believer that recipe formulation has a lot to do with a great beer. mixed with controlled fermentation, well treated wort during production and fresh ingredients, its something to strive to get right.
The key word is "BALLANCE"!
print that word out in big bold letters and hang it up where you brew! make it your new moto!
To achieve it time after time in your beers will take time and experience. Im with newguy too. taste your malts........ all of them. You will get a feel for what effect they will have on the beer. Chew some crystal malt and then some amber malt. You will soon taste the difference between the sweet of the crystal and the dry buiskety maltiness of the amber.
As said..... your base malts will differ too, taste them. pilsner malt will be sweeter and cleaner, ale malts more biskety. Munich malts have a toasty malt character.
I have always tasted my grain when brewing, or if im board and feel like comparing malts. I have developed a bit of a memory for what they taste like in my head now and a bit like food, you get to know what tastes good with what.
There are some basic rules for base malts that you can follow for a start and then let experience and your taste buda guide you.
base malts......... obvious..... use as much as you want.
Munich malts........ Using 1 or 2% is a waste of time. Munich 1 can be used as a base malt and will make a really toasty bready sweetly malty beer. ITs great in most ales at 10 to 20% grist to increase the malt profile of the beer. Munich 2 is the same but a bit darker so use 30% less of it as you would minich 1.
Caramalts are a light crystal type malt that add a sweet honey like maltiness to the beer. Use them up to 10 in light lagers and ales to boost the malt profile and add head retention.
Cryatal malts are sweet and malty. there are 3 main groups...... l
light to standard, 100 to 150 EBC, use at around 6 or 8 % in a porter of english bitter, can use more can use less.... up to your tastes.
dark, 250 EBC, deep crystal flavour, because its darker, use less that the lighter stuff.
Caraaroma, german dark crystal...... 450 ebc i think. use to a few % for deep dark fruit flavours and a red hue to the beer.
Amber and brown malts. These are drier and toastier. They offer no real sweetness and should be used with restraint. I recomend you use on or the other in a beer and keep it at 1 or 2 % Its great when mixed with a good dose of crystal malt to ballance the sweetness of the crystal. There's that word
Dark malts. the darker they get the less you need. I have a general rule with brewing stout....... keep the dark roast malts under 10% or grist.
For a porter, use around 8% crystal, 5% chocolate and 1% black malt with a good ale malt as the base. See the ballance? the sweet of the crystal will ballance the dry roast of the choc and black malts.
another good trick if you have no ale malt is to use 50/50 pilsner malt/Munich malt. Great base for a porter or stout.
There are so many malts......... wheat malts, smoked malts, things like maize, flaked barley, carapils, the list is a long one but with time you will work it out.
If you ever want confirmation of amounts in a recipe, just post them of PM me if you want.
A work mate of mine was brewing a beer from a recipe in a book. HE asked me what i thought of the recipe and as it was basicly just base ale malt and lots of roast barley and black malt, i said it would be harsh roasty and a bit astringent.
He brewed it anyway and i was right!. It needed a good 8 to 10% 145 EBC crystal to add a weetness that would hold the roast up in the beer.
Think of it like lemonade....... lots of lemmon on its own will be yuck but add some sugar to ballance the sour and what have you got. YUM!
Sorry for the rant but i am quite passionate about this aspect of brewing.
cheers