No point in buying walking boots unless you are intending to walk on very rough ground, and no-one can advise you on which boots to buy, as what suits one pair of feet assuredly does not suit another.
If you are going to buy trekking boots or shoes, most people prefer a half size larger than they normally wear, as they can then wear two pairs of socks, one thin pair and a thicker pair on top to mimimize the risk of blisters.
This, however, just does not work for me, for I cannot walk if my feet get too warm, so I have always worn shoes rather than boots or in warm weather trekking sandals.
Start off with a pair of shoes or boots that you have and that are comfortable, and do not walk too far at first, gradually increasing the distance you walk.
Do you usually use a walking stick? If not on asphalt you won't need one, or find it anything but a nusiance. On rough ground, a stick my well be advisable, but a good old fashioned staff made of a straight branch of a tree, debarked where you hold it, and shoulder high will prove far better suport.
As to clothes, wear whatever you are comfortable in, but trousers are not a good idea at all IMO as you expose far more of yourself every time you need to wee than you do if you wear a not too tight skirt.
Rain-proof clothes - yes, essential if walking in the west of Scotland, but otherwise a light weight raincoat or cape should be enough.