Celt covers a LOT of time and area. To get an impression, you need to narrow it down.
You could be anything from a near naked man with blue paint and a spear to A fully plate armoured knight in imported Millanese full plate.
Time period - wealth - localised/ influenced - travelled - location
Pre-Roman - Roman - Iron Age - Transition/ Viking - Post Norman - Post Plauge - Renaissance
Are you portraying a wealthy man or a basic soldier
Are you portraying a localised persona or a heavily influenced person - Romanised/ Vikingised/ Adpoted imported plate etc
Are you a stay at home local man or have you travelled to pick up external influences and ways
Insular Celt, Continental Celt