The Obama administration on Monday showed off the federal Web site, Healthcare.gov, where millions of Americans starting Tuesday will be able to buy coverage under President Obama's health-care law, promising it will be open for business despite congressional battling and widespread reports of computer problems. People seeking to buy health insurance on the federal marketplace will be able to enter personal information, including their incomes and Social Security numbers; learn how much government assistance they might qualify for, if any; search for plans by price and coverage level; and then purchase a plan directly from the insurance company.