Get several quotes, dont be presurised into instant agreement by sales talk.
Do your research into what you can afford, what type of renewable energy products you favour. Ask as many questions about your options as you need to satisfy you.
Be aware that the Renewable energy is a fast growing industry and along with companies that are in it for the short ride and too make a killing whilst its available.
Check out any chosen companies for MCS acreditation and sustainability, the larger companies often have larger overheads and thus higher charges, although experiance has shown that some smaller companies can also over inflate the charges made. Research the make and model of proposed panels and inverters being used. Many blogs or chat rooms on renewable energy web sites will offer tales of wo and stories of sucess to guid your research.
Check with your local area electricity infrastructure owner to esure their agreement with your plans befor signing any instalation agreement. Depending on your plans and ideas, dont forget your local planning department if they are required to know, research on any blog forum again will tell you if planning consent is required with your plans.
Ask to view similar instalations errected in your area, talk to the customers concerned about the system operation and the installing company. Once you've chosen an installer, get in writing what has been agreed and terms of payment.
A good company should split the payments in instalation stages and agree to complete the necessary FIT paperwork after commissioning of the instalation and prior to final payment.