I did introduce a beaker at around 7 months but at first I only ever offered water in it, just while dd was learning to use it. The best time to offer it was during solid feeds - offer and then leave it on the highchair tray so they can examine, play with and maybe attempt to use it. I never attempted to put milk into a beaker until she was over 1 year old as I knew her milk intake would drop and she wasnt a big milk drinker in the first place. She was off bottles by the time she was 14 months with no major traumas.
I found the best cup to be a tommee tippee cup with a flip up spout which is freeflowing (ie NOT non-spill as these are harder to use at first as baby has to work harder to get the water out). I didnt check where you are from but this is what the cup is like:
http://www.tommeetippee.co.uk/index.php?p=31Once dd was ok with these ones she moved to the avent magic cup which has a non-spill valve but by then she knew she had to suck to get anything out.
http://www.avent.com/uk/en/toddlerfeeding_magic_cup.phpWhichever cup you choose, your lo will need plenty of time and practice before they start taking much from it - the key is to persevere, preferably sticking to one cup that is easy to use - try it yourself to be sure your lo will be able to get anything. There was one cup we bought that I found hard to use so I dont know how a child would be expected to use it successfully!!
hth
Seona