To change your clthes for Sims 1, u first have to buy a wardrobe from the shop. Then place it somewhere in your house so you can use it. Click on the wardrobe, then select change clothes. It will then take you so you can change clothes.
To change the type of outfit ie. swimming costume, pjs ect. click on the dresser and select "change outfit". To modify the outfit click "plan outfit". This will take you to the create a sim mode....
Once you create a sim you have already chosen their clothes. If you have open for buisness you can go to community shops and some with sell clothes. You can then take them home and plan your clothes...