I'll give you a hint, if I can remember how to correctly:
Decide if you want to design your network by either number of subnets or number of hosts on each subnet.
Remember that there are eight bits in each OCTet. Decide how many bits you need to accomidate the number of subnets or hosts. Say you need six subnets, 1 bit is not enough (only 2 possibilities) 2 only 4 possibilities not enough... When you decide what you need save that number of bits at the start of the bits you have to play with and reserve those for your "subnet bits", the rest are hosts. Remember that you can have a net address of 0 but not a host address of all 0's (this is reserved for the network address), or all 1's (this is reserved for the broadcast address). I realize that this probably isn't the greatest explanation and some of the people here can run circles around me but there you have it! Just practice, practice, practice subnets and you'll gradually get better at it.