ERA-Interim overlaps and extends the ERA40 Re-Analysis. The dataset contains a large number of fields, at varying temporal resolution, but spatial resolution seems to be 0.75º x 0.75º for the data fiels we've been interested in.
Sign up at http://apps.ecmwf.int - it can take a while to get a response. Then sign in, and look for Forecasts, Datasets from the top menu, and then Browse Reanalysis (as opposed to forcasting). From there, you have a choice of various datasets including ERA-Interim. Click Data Access to get to the download page. Maybe the best thing next is to select the field you want from the bottom, and see what fields get disabled above it. Then select your dates and times.
The Steps choice is a bit interesting - see here. It differs depending on whether the fields you are requesting are instantaneous (like temperature), or accumulated (like precipitation). For instantaneous, you can use it to get intermediate timesteps, using ERA Interim's own internal forecast model; for example, if you want temperature at 3pm, you can select temperature at 12pm, with step 3 (+3 hours). For accumulated, the step I think is the length of accumulation, so to get daily precipitation, you'd want a reading at 00:00, with a step of 24, or two readings at 00:00 and 12:00, with a step of 12 hours, and add them.
After you select everything, you can retrieve the data in Grib, or NetCDF format. I'm going to go with grib. The grib is assembled by their MARS server, and you leave your browser open, and wait for a download link to appear. It could take some time depending on how much data you've asked for.
Grib files can be read into Java with the jgrib.jar library - but perhaps an easier, more universal way, is to just convert them to CSVs, using DeGrib.exe - it's a Windows executable. Sorry Mac/Linux users - source code is there - mail me if to request binaries.
For the 2T files, I use:
degrib _mars_filename_123456.grib -C -msg all -Csv -makeStyle 2T_%V.csv
and I get CSV files with names like 2T_201504180600.csv - for the 18th April, 2015, at 0600 hours. The CSV files is nicely labeled with columns, x, y, latitude, longitude, and the field value, which for 2T, is in Kelvin. Check the ERA documentation for your fields.
Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Hólm, E. V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N. and Vitart, F. (2011), The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Q.J.R. Meteorol. Soc., 137: 553–597. doi:10.1002/qj.828