When switching to CiviCRM from another system, what do I need to consider?
Migrating Existing Data

Migrating data (and processes) from a previously used (CRM) system is not a routine task. It will probably involve discussions, how existing workflows can be represented in CiviCRM - or what might even be changed.

Data can be imported into CiviCRM in the csv format. There is a field for "External Identifier" in the CiviCRM database that can save the connection to the identifiers of records in the old system.

General Aspects to Consider

There are a number of different aspects to consider. The CiviCRM User Guide actual provides a lot of good information about this.

Cost

see What is the price of CiviCRM?

CiviCRM is open-source and therefore itself available for free but you will need to pay hosting it on a server. Also, if you're not a techie you may need to pay for an expert for installation and support.

Hosting

see CiviCRM User Guide: Hosting

If you take care of administering the system yourself on a webserver it will be cheap. If you lack the technical expertise and need a stable system, consider getting a complete hosted CiviCRM system from a provider.

Change Management

see CiviCRM User Guide: Transitioning to CiviCRM

Don't forget to get the people from your organisation on board with your plans.