This is off-topic but if you want to start small go with EC2/RDS (database) and then as you grow you can use their other services to scale up. Services like Autoscale, ELB, Cloudformation, DynamoDB, etc.
You can also start small with their Elastic Beanstalk service assuming that your application is in one of the supported stacks. Once you deploy to it, AWS takes care of all the scalability.
Another alternative is to use a service like Heroku which autoscales as your needs (and happens to run on top of AWS)
Finally, you can also look at other hosting places like Rackspace, Digital Ocean, Joyent among many others.