The only way to obtain useful data is to have well prepared samples.
Samples of the required material should be dissolved in an appropriate DEUTERATED solvent. The deuterated solvent serves two purposes. First it acts as the lock material and secondly, if you are running a proton spectrum, it removes the presence of a large solvent peak.
Use an appropriate concentration.
For a proton spectrum on the majority of organic materials of molecular weight of under 500 one needs between 3 mg to 5 mg dissolved in 0.7 ml of a deuterated solvent. Since 13C is of lower natural abundance and lower sensitivity one needs more material to be able to obtain data in a reasonable amount of time. In this case one should use a minimum of 25 mg dissolved in 0.7 ml of deuterated solvent.
Solutions samples should be just that, solutions. There should be no particulate matter in the NMR tube. If necessary filter your samples. Gel like samples will be difficult to shim and give very broad lines.
Instructions for the Bruker Instruments:
Instructions for the Varian Instruments: