In many cases you will want to connect at least some of your populations with random sparse connectivity. To do so you simply instantiate an object of type SparseConnection which does exactly that. SparseConnection comes with a set of tools to directly add Hebbian assemblies or other simple structures into the synaptic weight matrix. However, if you want even more structure SparseConnection can import MatrixMarket files, which conveniently allows you to build complicated connectivity structures in MATLAB or Python and then load them into your simulation.
If you want to simulate plastic synapses simply use a Connection object of type STPDConnection or for instance TripletConnection for triplet STDP (after Pfister and Gerstner). You want short-term plasticity instead, then STPConnection is for you. It implements the Tsodyks-Markram model. You want to define arbitrarily shaped STDP windows and do not care about all-to-all spike interactions? Use PairInteractionConnection. In most cases you will want to write your own plasticity model – have a look at this tutorial which will help you to get started writing_your_own_plasticity_model.