The estar com construction is much more common than the transliteration to be with in English (although it does also mean that).
In many cases it is better translated idiomatically as have. This tends to occur more for something temporary than fixed ones; much like ser/estar, but it's a less concrete rule. Generally ter is used to indicate possession; as a general rule, where an English "I have..." couldn't be logically substituted for "I possess...", it's more likely that you'd use "Estou com..." than "Tenho...".
In many cases "estar com" will be much more common, but ter isn't wrong per se, just a little stilted and old-fashioned sounding.
For things on your person:
"Você está com a chave?"
"Do you have the key?"
For things you "have" that aren't yours.
"Ainda estou com o livro dele"
"I still have his book"
Many adjectives in English only have a noun in Portuguese, for example, in Portuguese you cannot "be thirsty", only "have thirst".
For feelings or emotions expressed with a noun
"estou com medo."
"I'm afraid." ("I am with fear" > "I have fear")
"Estou com esperança que vai dar certo"
"I'm hopeful that everything will work out"
For descriptions of temporary characteristics which use nouns (usually these are achieved with adjectives in English).
"Estou com fome"
"O meu celular está sem bateria"
"My phone is out of battery"
(This applies to Brazilians; I think ter might be more common in Portugal.)