depends on what you are growing in. first grow? there are many you can use. FloraGrow Bloom is used alot here but you can use GH or any of the others. If you have been vegging for 5 weeks, you can start nutes, start with 1/4 the strength on the label every other watering or something like that, increase dose as you see them taking the nutes. If they start to burn, back off. If you are using MG or any pre nuted soil, chill and really go slow.
my 2 cents.