I am doing the accounts for some other people for the first time in manager. And I have been up all night worrying. I was taught debit assets and credit liabilities.
When I do a journal in manager I credit sales and debit expenses. I am assuming it does the right thing and this is how I do it I don’t have to put a negative sign in front of the amounts?
I am probably over thinking I just want it to be correct.
Be happy. Yes, you are overthinking. The only time you really need to know whether to debit or credit is for journal entries, where you list both accounts and have the flexibility to enter things under the wrong columns. 
If you stick to Spend Money, Receive Money, Transfer Money, sales invoices, purchase invoices, and so forth, Manager figures out whether to debit or credit. For example, if you spend money, Manager is going to credit the account you spend from. No positives or negatives needed.
As you bounce around between the various screens and drill down into Summary entries, you will see negative signs as appropriate, even though you did not enter them.
Last point: you should avoid journal entries as much as possible. Of course, the shortcuts are equivalent from an accounting viewpoint, but all the good features of the program are elsewhere. In fact, some things (like bank transactions) you currently cannot enter via journal entries.