たい and ほしい: wanting actions and things
Learn the basic contrast between たい for an action you want to do and ほしい for a thing you want. Expressing your own wish is also different from making a request.
たい for an action you want to do
Remove ます from 読みます and add たい to make 読みたい. 日本の小説を読みたいです。 means 'I want to read a Japanese novel.' Do not attach たい directly to the dictionary form as 読むたい.
たい changes like an い-adjective: 読みたくない and 読みたかった. The desired action's object can take を or が. Learn complete examples rather than assuming that only one of these particles is possible.
ほしい for a thing you want
新しい辞書がほしいです。 means 'I want a new dictionary.' Start with noun + が + ほしい. 新しい辞書を買いたいです instead expresses a wish to perform the action of buying one.
The negative of ほしい is ほしくない and its past is ほしかった. Not wanting a thing and not wanting to buy it can overlap, but they are not always equivalent. Keep the desired object separate from the proposed action.
Whose wish, and is it a request?
In ordinary statements, たい and ほしい directly express the speaker's own wish. For someone else's wish, reporting what they said, as in 本人は行きたいと言っています, makes the basis of the statement clear.
At a café, 水がほしいです communicates that you want water, while 水をください directly requests it. Do not treat wish expressions and requests for someone else to act as mechanically interchangeable.
Check: How do you say 'I want to go to Japan' and 'I want a new bag'?
Show the answer and why
Use 日本に行きたいです。 and 新しいかばんがほしいです。 The first concerns an action; the second concerns wanting a thing.