actually, prepositions do not reflect gender in Portuguese, only when they are combined with an article. you have four possible combinations of gender/number in Portuguese: feminine/singular, feminine/plural, masculine/singular and masculine/plural.
now, you have definite articles (corresponding to 'the' in English) and indefinite articles (corresponding to 'a'/'an' in English). the first one is 'a' for feminine/singular, 'as' for feminine/plural, 'o' for masculine/singular and 'os' for masculine/plural; as for indefinite articles, they are 'uma', 'umas', 'um' and 'uns', respectively.
then, prepositions: 'to' can be roughly translated as 'para', 'from' as 'de' and 'at' as 'em'. i say roughly because it depends on the context they are used in. now, notice that when you combine some prepositions as 'em' and 'de' with articles in Portuguese, you get a different form, e.g. 'em'+'a'='na'; 'em'+'as'='nas'; 'em'+'o'='no'; 'em'+'os'='nos'; 'de'+'a'='da'; 'de'+'as'='das'; 'de'+'o'='do'; 'de'+'os'='dos'.
finally, possessives! i think you should wait for Tereza's commment on this, because i speak European Portuguese as a native language and there are some diferences from Brazilian Portuguese. also, she can refer you to some lesson in Rocket Languages that i do not have access to. anyway, generally grammars list possessives as follows:
for feminine/singular: 'minha' ('mine'), 'tua' ('yours'), 'sua' ('his'/'hers'), 'nossa' ('ours'), 'vossa' ('yours'), 'sua' ('theirs'); feminine/plural: 'minhas', 'tuas', 'suas', 'nossas', 'vossas', 'suas'; masculine/singular: 'meu', 'teu', 'seu', 'nosso', 'vosso', 'seu'; masculine/plural: 'meus', 'teus', 'seus', 'nossos', 'vossos', 'seus'.
i hope this helps and that it's not too confusing!
good luck :)