Agree that its not a stupid question at all. Mothers of big babies often get told that their milk won't be enough to sustain their Lo, erm so how did Lo get that big? 
Equally mums of small babies are encouraged to wean early to encourage baby to "get a bit extra" when the BM is far more calorific than most fruit and veg, which is all you can basically give them before 6 months.
Agree too that there is a lot of contradictory advice regarding weaning. If you bf then wait to 6 months and feed your baby what you eat, the baby food manufacturers will make very little, if any money out of you, so its in their interests to blur the issue.
If you do choose to wean before 6 months then 17 weeks is the absolute earliest, their is evidence that weaning before this time can be harmful.
Have a read of this on starting solids as it gives you the signs of readiness.
This, this and and this are all good articles]]
Do you have a lll group near to you? They sometimes have talks on weaning and the leader will also be able to talk to you about your concerns.
In some areas the nct have weaning courses. These have a few but there is a reduction for families on low incomes and you get all the latest evidence based info, plus a chance to meet mums with babies around the same age 