Start by visiting
http://Assynt Tourism Group-Discover Assynt
This will give info on accomodation - book early, there isn't a lot.
Click on the 'Fishing' tab for links to websites of local angling groups; all the gen on locations, permits, boats etc.
Another useful site is
http://Angling in Assynt Research
Bruce Sandison's book is essential, as is Cathel Macleod's 'Trout Fishing in Assynt'.
If you've never fished lochs you'll need to gen up on tackle, tactics, flies etc and Stan Headley's 'Loch Fisher's Bible' will be helpful.
Get the OS map and be amazed at the amount of water available, more than 200 lochs; you'll be spoilt for choice.
If you're thinking of fishing lochs Assynt and Sionascaig be advised that in the event of a cold Spring these lochs won't begin to fish until well into July, so have a few alternative plans in place. I'd advise seeking out the small sheltered lochs of North Assynt. Inevitably the weather will have an influence on your choice - it can get very windy up there.
This area is truly an angler's paradise, you'll want to return time and time again.
EDIT - sorry about the links, they don't seem to be working. Just use google.