First you need to decide what is the main function of the network. The location of the users, and the servers is largely decided by the layout of the building, and the data cabling to them is probably already in place. There are going to be internal resources (such as servers and printers) that the users need to access, as well as things like Internet access. There may also be IP telephony.
The users are also usually divided into different types of user, general office workers who will be fairly light users of the network, finance who often have their own servers and use it more, you may have heavy network users such as video editing, it will depend on the business type.
Once you have an idea of how the network will be used, then you can start on the design. The recommended method is to have an access layer, with fairly cheap layer 2 switches for the users to connect (usually at 10/100 mpbs), with uplinks from these switches to the distribution layer (probably 1Gbps), and these link to the network Core layer (redundant 1Gbps or multiples of this, or 10Gbps, links). This provides a hierarchical structure that is easier to expand in the future. In a smaller or medium business, the distribution and core will collapse to the same layer.
Plan the VLANS to map to the different user groups, and place any servers that are specific to a group in the same VLAN if appropriate. Some advise that servers go at the access level, I think they are better placed at the core if shared among different groups. This gives a shorter network path to the resources, and also gives higher bandwidth and availability. you will also need a separate VLAN, and also implement quality of service (QoS) for the IP telephony if it is there. In any case yo need to make sure that any switches support QoS for any future network resource that needs this functionality.
The core switches need to be fairly big powerful devices, and usually with layer 3 capability. There will be expensive, but also try to build in redundancy by having more than one (or more than one supervisor blade and multiple power supplies), to ensure it is available all the time.
I hope that gives you some idea of the planning process. It is usually constrained by location and always by budget ! It is also sometimes constrained because the customer wants a particular vendor to supply the equipment, and not all of them support all the features you may want.
If possible get as much information about how the network is going to be used, and base the design around that. Try to build in some spare capacity, for any future expansion of the company, and make the design as flexible as possible to cope with any changes. Also try to get as much time as possible to do the design, it has to work for a long time, so it needs to be as ‘right’ as possible when implemented.