A negative test can be wrong, but a positive test no matter how faint is very unlikely to be wrong.
If the line is faint it could be due to several things a) your urine was very dilute (loads of water) b) you're not very far along and haven't produced much of the hcg hormone c) its not a very sensitive test.
On the test box it should say either how soon it can be used (first response is 3days before your period is due) or it should list the sensitivity like 50iu/l or 20iu/l (international units pre litre) the lower the number the more sensitive the test is.
If you have had a faint positive test wait a few days and then re-test using your first morning urine(more likely to be concentrated), but yes if you got a positive result chances are that you are pregnant.