Dieser means "this" and takes the same endings as der/die/das - dieser, diese, dieses. Welcher asks "which" and follows the same pattern. To ask a price, use Wie viel kostet das? for one thing and kosten for several.
Examples
- Wie viel kostet dieses Hemd?
- How much is this shirt?
- Welche Größe haben Sie?
- What size do you have?
- Wie viel kosten die Schuhe?
- How much are the shoes?
The mistake to avoid
Wie viel is two words for an amount; wieviel as one word is an old spelling.
Where this fits in
Food & ordering, German A1: Order at a café or restaurant, ask for the bill, and talk about basic food and drink.