You don't have to hire a tax consultant, there is a number of companies who sell software (installable or web-based) that helps you do it by asking for all relevant data interview-style. These typically cost between 15 and 25 EUR. I'm not sure whether any of them are available in English, but if you can read German well, you should be OK.
taxback.com is in English, but to be honest it looks a bit dodgy to me.
Now for your questions:
are there some tricky fields (lines) that after filling in my taxes will be counted higher?
This is rare, at least for employees you nearly always get something back.
are there some tricky fields (lines) that after filling in my taxes could be counted lower?
Not in general. Marriage is mentioned below, and otherwise it's all about individual deductibles. Ah, one important factor: if you have investment income and have not filed a Freistellungsauftrag with your bank, you can get some of the taxes by filling out the "Anlage KAP" form with data you got in the Jahressteuerbescheinigung from your bank.
are there some flat-rates (Pauschals) that I could get advantage from?
Absolutely. As an employee, the biggest factor is the Werbungskostenpauschale of (I think currently) 1000 EUR for general work-related expenses, which will be accepted without proof. If your expenses are higher than that and you file individual expenses, there are flat rates for work-related moving and for commuting distance.
is it better to give a tax return together with my wife (who was only a girlfriend in 2013 living with me in one household) or to give it separately?
It's not possible to do a joint tax filing for the time before your marriage. What you should consider is to apply for a different tax class from now on, if one of you earns significantly more than the other.
when separately, do I have to fill her information in my tax return or can I just pretend there is nobody else in my apartment?
As far as taxes are concerned, unmarried roommates are treated completely separately with one exception: only one of you can deduct service charges included in your rent. You have to get a Nebenkostenabrechnung from your landlord, and service charges, i.e. janitor, gardener, etc. should be marked separately. But this may not be worth bothering with, usually it results in a tax return of maybe 15 EUR.
is there any guide in English that could be of help with filling in the tax return form?
I couldn't find anything that looked really useful in a short search.
