If you mean that the subject is "in/inside something" then you use -ssa/-ssä. If you mean that the subject is "on something" then you use -lla/-llä. For example. A book is on a table. - Kirja on pöydällä (pöyta - a table). Now I am in Finland. - Nyt minä olen Suomessa. Though this rule has its exceptions (e.g. some geographical objects require usage of external locative cases - Venäjä [Russia], Tampere [a city in Finland], all the names that end with -järvi, -mäki, etc.).