Getting tooled up
Good tools always make the job easier and will pay for themselves over and over again in time and frustration saved. Making do with a pair of mole grips just leads to frustration, a damaged bike and skinned knuckles.
For most routine jobs all you need is :-
Spanners 7 to 15 mm (1/4 - 3/4")
Allen keys 1.5 - 10 mm (1/16 -3.8 ")
If your bike is a trusted friend of 15 or more years standing it may well be put together with Imperial nuts and bolts so check before you buy.
If you want to get more adventurous and really pull the thing to bits add :-
Head set spanners
Bottom bracket spanner
Given that lot and a good bike book or service manual (Haynes do one for mountain bikes) you should be able to tackle most jobs on your bike.