Hey there Frampi, firstly assess what gear you will need, everything you "should" need should fit inside a 60-65ltr pack. GO AS LIGHT AS POSSIBLE, avoid packing loads of tinned goods etc, there's plenty stops along the way to re-supply. Take advantage of the "honesty shops" dotted about along the route, drop a couple of quid in, help yourself to coffee/tea/ bottle of water and cookie, have a wee break.
Seriously, excess weight is a real issue here, after balmaha on to Rowendennan you'll encounter some pretty harsh ascents and descents, harsh on the ankles. I've stopped for a pint in the clansman in Rowendennan and seen a good number of folk throw in the towel at this point due to sprained ankles.
I'd say, get yourself a copy of the footprint map before you set off, plan your stops for supplies etc. Take it easy and enjoy it, don't try to do massive distances each day. Take some smidge, midgy repellent and a head net (its a must). Happy trails