If what you want is to perform an action, you must modify the verb in question, rather than use the adjective ほしい. To do this you must transform the verb into an い-adjective by placing たい at the end of the stem of the verb.
|I want to drink beer.||私 は ビール が 飲みたい です。watashi wa biiru ga nomitai desu.|
|(I) want to learn Japanese.||日本語 が 習いたい です。nihongo ga naraitai desu.|
|(I) want to learn Japanese.||日本語 を 習いたい です。nihongo o naraitai desu.|
|What do (you) want to eat tomorrow?||明日 は 何 を 食べたい です か？ashita wa nani o tabetai desu ka?|
If the verb has a direct object, you can now choose to use either を or が as a particle. The reason が is possible, is that the verb has been transformed into an adjective. When you use が with a verb in the たい form, it can be used to express a personal preference or desire, and can sometimes come off as more polite because it may sound less assertive than を.
After the verb has been modified in this way, it behaves just like any other い-adjective when you want to conjugate it into the negative or past tense.