Related technologies are like CRM, SCM, BI et al. First go for ERP implementation. Go for big bang approach or parallel execution depending on your confidence level. Better eat the elephant by bytes.
During implementation of ERP package ( which may take 3-12 months depending on organisation size and also number of modules ) , go for change management, continuous training and also consolidation of changes. These are very important for the success of your implementation. If not done properly there are many examples of costly failures and re-implementations.
After implementing the core ERP solution go for a Application depending on your industry. For example if you are in Banking industry which is customer facing then go for CRM package first. If you are in manufacturing go for SCM. After that go for Data mining and warehousing and Business Intelligence.