Assuming you have a recipe to work with (if not we can explore that later on) but for now just find a recipe in a brewing magazine, the internet, or use one of mine I have supplied with our lineup of beers.
For now let's talk in 5 gallon batches, once you get the hang of things you can make larger batches as long as you have the equipment to do so.
Typically a 5 gallon batch is going to have approximately 10 pounds of grains. Usually made up of your base grains and some specialty grains or even some adjuncts such as flaked corn or rice.
I like to start by filling my 20 gallon hot water tank to about 3/4 full and heat it up to about 160 degrees. For your 10 pounds of grains you will need about 1-1/2 quarts of water per pound of grain. So we are looking at around 15 quarts of water, let's just ound it off to 16 quarts which equals 4 gallons. So empty four gallons of the 160 degree water into your mash tun and slowly add your 10 pounds of grains. Stir the mash as you add the grains so you don't have a bunch of clumps. Check your temperature of the mash it should be around 150 - 155 degrees. If you are below 150 degrees turn your burner on and stir for a few minutes until you get to around 152 degrees. Then put your lid on and check it every 15 minutes or so while stirring. After an hour or so you need to do a starch test.