A proper financial base is essential for a good iGEM start. It is very important to raise money for your team.
You should start thinking about funding very early. You have to have enough money to pay for registration fees, travel costs, Jamboree Participation, lab materials, machines, etc. The way you will find your funding depends on the particular situation of your high-school (for instance country, city). However, we will try to give you some general ideas on how to approach this problem.
Estimate all the expenses at the beginning, then multiply by 1.5. To get an idea of costs, check out some wikis of previous iGEM teams. One big aspect is the size of the team, because of the travel and registration expenses. You also have to consider if you have all laboratory equipment, and consumables or if you need to buy them.
To get some information on how to find a sponsor, contact existing iGEM teams in your area and ask them how they accomplished that task. Contact the companies you are going to order from; they might propose you a discount or offer you some products for free. Find out about possible governmental help for science projects at the high-school level. Present your project to local labs, foundations, and companies. Local labs might provide you a space to work and materials, while companies and foundations can provide financial support.
This should give you a basis to support your iGEM team, good luck!