Greetings, FEaereBrew. You do NOT have to make a starter. For a standard 20 liter batch a package of liquid yeast may not be enough, so it is recommended to make a starter to increase the number of yeast cells you pitch. If you are making a smaller batch then you can pitch the whole package with no starter, or you can pitch part of it if you wish. It depends on the beer you are making and the age of your yeast packet. BeerSmith uses a default pitch rate (changeable via Options->Yeast Starter) of 0.75 million cells/(ml-Plato) for ale, 1.5 for lager and 1.0 for "hybrid " beer. Many other sites recommend 0.75 for ale with Original Gravities less than 1.060 and 1.0 for ales with Orignial Gravities above that. If you have a brand-new package of yeast with 100 billion cells and a starting gravity of 1.040 (=10 degrees Plato) in 11 liters then your pitching rate would be 10^11 cells/(11000 ml*10 Plato)=0.91 million cells/ml-Plato, which is plenty. If your yeast have degraded until they only have 85% viability and your OG is 1.055 (13.6 Plato) then your pitching rate would be 0.57 and that is on the low side so you definitely want to pitch it all.
Look at your gravity and the number of viable yeast cells that BeerSmith predicts based on the age of your packet and decide if you want to pitch all of it or less, but I would guess that you probably want to pitch it all. Unless your yeast is very old and your beer is very high gravity you should not need to make a starter.
--GF
