There are so many travel agencies, tour operators in Cusco wanting to take you to Machu Picchu. After many days to looking around and trying to decide, we found out from a few people that the best and cheapest way to go to Machu Picchu is to obviously do it on your own.
We bought train tickets from Cusco - Aguas Calientes (USD98 return on backpacker class). A cheaper option is to bus to Ollantaytambo and catch a train to Aguas from there. Or just bus it all the way, but it won't be as fun.
Stayed a night in a lovely hostel in Aguas Calientes (roughly USD10 depending), and caught the earliest bus at 5.30am to Machu Picchu (USD14 return, or if you are up for it, you can walk back to Aguas and save yourself USD7). The earliest bus is advisable if you want to hike up to Huaynapicchu. They only give out 400 passes a day, so if you want to, you'd have to go straight to the entrance booth to Huaynapicchu once you enter Machu Picchu.
It shouldn't be a problem buying your entrance ticket to Machu Picchu in Aguas (62soles for a student ticket). We got ours the day before at the INC in Aguas. We also hired a guide when we were in Aguas, and we thought it was 20-25 soles well-spent. He was very informative and helpful, nice, kind, useful.....blah blah blah. Definitely a good idea and one of the best guides we've had.
However, I was told that Machu Picchu closes for the whole month of February due to the rainy season and for maintenance. I'm not 100% sure, but you might want to check this out.
All the best.