I am trying to saturate a transistor , a BC547 A , but I am having a hard time figuring out something.What hFE should I use to compute the base and collector current to make it turn on fully?
Here is the datasheet
If the collector current is between 10mA and 100mA use Ic/Ib = 20, because the limits are specified on the data sheet for that ratio. You can expect Vce(sat) between a maximum of 600mV and 250mV depending on collector current. If the collector current is much larger than 100mA or extremely tiny (nA) you may want to select a different device or increase the base current respectively.
Estimate Ic(sat). Then take hfe to be about 20, and calculate your base drive. Be careful not to overheat the transistor; look at VCE(sat) * Ic(sat) to get an idea of power dissipation.
Look at VCE to estimate how deep into saturation your transistor is. TIP: Look at the VCE sat in the electrical characteristics. That tells you what Ib takes the transistor into saturation for a given Ic. If your Ic is less you can get away with lower Ib and hence slightly lower overall power.