Double the required amp rating of whatever you’re plugging in absolute minimum.
Only use the constant amp rating of the inverter not the peak amps.
Get the right voltage as mentioned. 
For microwave, read the back of the device to see what it actually uses. Eg a 800w microwave could actually pull 1200w as 800w will be the cooking wattage not what the whole machine takes.
Modified sinewave is alright for basic stuff. Pure sinewave is best for anytjibg with complex electronics like computers, microwaves and some modern TVs.
Go buy a decent brand nit a cheapo off eBay. If its cheaper than the rest theres a reason.
Finally it’ll need hard wiring professionally not via jump leads or cig lighter! Good way to burn truck electronics out.