What I need to do before I start a new ERP implementation?

The First thing that we need to consider is understanding your business and your business need.  ERP is just enabler to your organization.

Without understanding your businesses will be a danger to the intended system where all investment can be just a waste and becoming a white elephant.

Each ERP system has their own strengh and weakness, thus by understanding your business and business requirement you may decide on level of ERP system requirement that suits your business need and its intended useful life without compromising organizations cashflow and be prudent.

I agree with Mr Mostafa, but assuming company has appropriate Org structure, DOA and P&P's are in place.

good chart account

high organization between department

Implementation have different phase.first of all implementors carefull studies the business process and gather the current and future business requirements of client.phasing allow you which module is needed to implement first for client business need.after that server installation phase wil be start.

Three main pillars should be considered keeping in mind budget and business needs

1- Technology provider (Oracle, SAP, others)

2- Infrastructure availability (hosting in own DC, cloud or other choices if available)

3- Operation (IT team, outsourcing,... etc)

Assuming that you already have answers for the above three main points, then ....

1- Decide which modules you are going to implement first according to business needs.

2- Create your implementation teams specialized in each module.

3- Chose from business units who are going to be your consultant in implementing each module.

4- Prepare your infrastructure accordingly (communication and servers).

5- Put general plan for implementation, then sub plans for each module considering dependency between modules.

6- Plan for the transition period between the old system and the new system. and parallel to this be ready with users training plan.