You prefer to see a house before taking it ? So you should book yourself a hostel for the first couple of days before arriving. There are dozens of hostels around Lisbon, we suggest a few hostels where you can stay for good price/quality ratio:
Recommended Hostels (10+) - Go to ESNcard.org (Link)
These hostels are locatel in Lisbon downtown close to metro, tram and train lines so you can have easy access to transportation. All are recommended by ESN and also with discounts on booking.
If you prefer to look for alternatives, consider using Coachsurfing or similar platforms.