Translation:I enter the hospital.
Depends on the verb - it wouldn't work with hairu but it would with say sampo suru or shinjiru. For example kooen wo sampo shimasu - I walk through the park. Kooen ni sampo shimasu - two possibilities - I walk to the park or I walk IN the park. Also anata wo shinjimasu - I believe you. anata ni shinjimasu - I believe IN you. Also certain verbs take specific particles as a rule/set construction.
はいり (hairi). You know by studying kanji and learning and memorising their different readings and also by context. はいります - means to enter, while いります means to need. Aside from having different kanji you should be able to figure out from context which is which - for instance, in this sentence いります would not make sense especially with the particle に showing motion towards or into. Books, apps, dictionaries, reading Japanese books and writing in Japanese regularly (eg. I often write notes in Japanese, shopping lists, reminders of appointments on the whiteboard on the fridge and chat with nihonjin friends on messenger and skype amongst other things) will all help you to learn and retain what you have learnt.
入り口 (いりぐち) and 入ります (はいります) - entrance and to enter respectfully, use the same kanji with different pronunciations (commonly called "readings" ie. ways of reading them). Then there is also another verb いります (要ります) - note the different kanji - which means to need. All kanji have multiple ways of reading them (a few examples - 正しい (正しい) - tadashii - correct/true, 証（あかし）-akashi - testimony - something spoken that is true (note kanji for to speak), 明るい (あかるい) - akarui - bright/clear, 明く (あく) - aku - to open, 明々(めいめい) - meimei - crystal clear
入る ・入ります will almost always be read as "hairu/hairimasu" for the intransitive verb "to enter/go in". "Iru" is an antiquated reading you will mainly only see in compound nouns and set expressions now.
Then there's 入れる ireru - "to put in", the transitive form of the verb.