You should feed your baby in whichever way you feel most comfortable with.
The current guidelines from the Government and the World Health Organisation are that you breastfeed exclusively for the first 6 months ie 26 weeks. That you do not introduce cows milk as a main drink until at least 1 year. The obtain the maximum benefit to health etc you should feed for 2 years +. This is what plenty of MNers have done - myself included. It doesnt feel strange to feed for that long - they dont suddenly grow up overnight you see .
Formula is a perfectly adequate substitute for infant feeding, however, it doesnt have the same 'benefits' to health from infancy to adulthood that breastmilk has been found to have. And breastmilk is FREE!!!!
If you want to choose a formula milk, make sure its one that you can find in your local supermarket AND in your local late night convenience store (things always run out at a bad time). Ignore any claims they have on the tin about being closer to breastmilk than other brands - they are currently unable to substantiate this